Display device and method for manufacturing display device

ABSTRACT

A highly flexible display device and a method for manufacturing the display device are provided. A transistor including a light-transmitting semiconductor film, a capacitor including a first electrode, a second electrode, and a dielectric film between the first electrode and the second electrode, and a first insulating film covering the semiconductor film are formed over a flexible substrate. The capacitor includes a region where the first electrode and the dielectric film are in contact with each other, and the first insulating film does not cover the region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention particularly relates to a semiconductor device, alight-emitting device, a display device, or a manufacturing methodthereof.

In this specification and the like, the term “semiconductor device”means all devices winch can operate by utilizing semiconductorcharacteristics. For example, an electro-optical device, a displaydevice, a light-emitting device, a semiconductor circuit, a transistor,and an electronic device may include a semiconductor device.

2. Description of the Related Art

In recent years, display devices such as a liquid crystal display deviceincluding a liquid crystal element as a display element and alight-emitting display device including an organic electroluminescent(EL) element (also referred to as an organic light-emitting diode, OLED,or the like) as a display element have been widely used. In order thatthese display devices have flexibility, the use of a flexible substratein the display devices has been examined.

As a method for manufacturing a display device using a flexiblesubstrate, a technique has been developed in which a semiconductorelement such as a thin film transistor is manufactured over a substratesuch as a glass substrate or a quartz substrate, for example, a spacebetween the semiconductor element and another substrate is filled withan organic resin, and then the semiconductor element is transferred fromthe glass substrate or the quartz substrate to the other substrate(e.g., a flexible substrate) (Patent Document 1).

PATENT DOCUMENT

-   [Patent Document 1] Japanese Published Patent Application No.    2003-174153

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide ahighly flexible display device and a method for manufacturing the same.Another object of one embodiment of the present invention is to providea non-breakable display device and a method for manufacturing the same.Another object of one embodiment of the present invention is to providea lightweight display device and a method for manufacturing the same.Another object of one embodiment of the present invention is to providean easily bendable display device and a method for manufacturing thesame.

Another object of one embodiment of the present invention is to providea highly reliable display device and a method for manufacturing thesame.

Another object of one embodiment of the present invention is to providea novel display device and a method for manufacturing the same. Notethat the descriptions of these objects do not disturb the existence ofother objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Note that other objectswill be apparent from the description of the specification, thedrawings, the claims, and the like and other objects can be derived fromthe description of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is a display device including,over a flexible substrate, a transistor which includes alight-transmitting semiconductor film, a capacitor which includes afirst electrode, a second electrode, and a dielectric film between thefirst electrode and the second electrode, and an insulating film whichcovers the semiconductor film. The capacitor includes a region where thefirst electrode and the dielectric film are in contact with each other.The insulating film does not cover the region.

One embodiment of the present invention is a display device including,over a flexible substrate, a transistor which includes alight-transmitting semiconductor film, a capacitor which includes afirst electrode, a second electrode, and a dielectric film between thefirst electrode and the second electrode, a light-emitting element, anda first insulating film which covers the semiconductor film. Thecapacitor includes a region where the first electrode and the dielectricfilm are in contact with each other. The first insulating film does notcover the region.

The first electrode is provided on the same surface as the semiconductorfilm. The light-emitting element is capable of emitting white light, forexample. A coloring layer may be provided so as to overlap with thelight-emitting element.

The display device may have a top-emission structure, a bottom-emissionstructure, or a dual-emission structure.

The display device is easily bendable. One embodiment of the presentinvention can provide a highly reliable display device which is noteasily broken even when a bending operation is repeated.

One embodiment of the present invention can provide a highly flexibledisplay device and a method for manufacturing the same.

One embodiment of the present invention can provide a highly reliabledisplay device and a method for manufacturing the same.

One embodiment of the present invention can provide a novel displaydevice and a method for manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are perspective views illustrating one embodiment of adisplay device.

FIG. 2 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 3A to 3C are a block diagram and circuit diagrams illustrating oneembodiment of a display device.

FIGS. 4A to 4C are cross-sectional views illustrating one embodiment ofa method for manufacturing a display device.

FIGS. 5A to 5C are cross-sectional views illustrating one embodiment ofa method for manufacturing a display device.

FIGS. 6A and 6B are cross-sectional views illustrating one embodiment ofa method for manufacturing a display device.

FIGS. 7A and 7B are cross-sectional views illustrating one embodiment ofa method for manufacturing a display device.

FIGS. 8A and 8B are cross-sectional views illustrating one embodiment ofa method for manufacturing a display device.

FIG. 9 is a cross-sectional view illustrating one embodiment of a methodfor manufacturing a display device.

FIG. 10 is a cross-sectional view illustrating one embodiment of amethod for manufacturing a display device.

FIG. 11 is a cross-sectional view illustrating one embodiment of amethod for manufacturing a display device.

FIGS. 12A to 12C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIG. 13 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 14A to 14C illustrate one embodiment of a transistor.

FIGS. 15A to 15C illustrate one embodiment of a transistor.

FIGS. 16A to 16B illustrate one embodiment of a transistor.

FIG. 17 illustrates a band structure of a transistor.

FIGS. 18A and 18B illustrate structure examples of light-emittingelements.

FIGS. 19A to 19E illustrate examples of electronic devices and lightingdevices.

FIGS. 20A and 20B illustrate one example of an electronic device.

FIG. 21 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 22 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 23 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 24 is a cross-sectional view illustrating one embodiment of adisplay device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it will be easily understood by those skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope of the present invention. Therefore, thepresent invention should not be construed as being limited to thedescription in the following embodiments. Note that in the structures ofthe invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description of such portions is not repeated.

Note that in each drawing referred to in this specification, the size ofeach component or the thickness of each layer might be exaggerated or aregion might be omitted for clarity of the invention. Therefore,embodiments of the invention are not limited to such scales. Especiallyin a top view, some components might not be illustrated for easyunderstanding.

The position, size, range, and the like of each component illustrated inthe drawings and the like are not accurately represented in some casesto facilitate understanding of the invention. Therefore, the disclosedinvention is not necessarily limited to the position, size, range, andthe like disclosed in the drawings and the like. For example, in theactual manufacturing process, a resist mask or the like might beunintentionally reduced in size by treatment such as etching, which isnot illustrated in some cases for easy understanding.

Especially in a top view (also referred to as a plan view), somecomponents might not be illustrated for easy understanding.

Note that ordinal numbers such as “first” and “second” in thisspecification and the like are used in order to avoid confusion amongcomponents and do not denote the priority or the order such as the orderof steps or the stacking order. A term without an ordinal number in thisspecification and the like might be provided with an ordinal number in aclaim in order to avoid confusion among components.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of a component. Forexample, an “electrode” is used as part of a “wiring” in some cases, andvice versa. Further, the term “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

Note that the term “over” or “under” in this specification and the likedoes not necessarily mean that a component is placed “directly above andin contact with” or “directly below and in contact with” anothercomponent. For example, the expression “electrode B over insulatinglayer A” does not necessarily mean that the electrode B is on and indirect contact with the insulating layer A and can mean the case whereanother component is provided between the insulating layer A and theelectrode B.

Further, functions of a source and a drain might be switched dependingon operation conditions, e.g., when a transistor having a differentpolarity is employed or a direction of current flow is changed incircuit operation. Therefore, it is difficult to define which is thesource (or the drain). Thus, the terms “source” and “drain” can be usedto denote the drain and the source, respectively.

In this specification and the like, the term “electrically connected”includes the case where components are connected through an objecthaving any electric function. There is no particular limitation on an“object having any electric function” as long as electric signals can betransmitted and received between components that are connected throughthe object. Thus, even when the expression “electrically connected” isused in this specification, there is a case in which no physicalconnection is made and a wiring is just extended in an actual circuit.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°.

In this specification, in the case where an etching step is performedafter a photolithography process, a resist mask formed in thephotolithography process is removed after the etching step, unlessotherwise specified.

(Embodiment 1)

In this embodiment, a semiconductor device which is one embodiment ofthe present invention and a manufacturing method thereof are describedwith reference to drawings. FIG. 1A is a perspective view of a displaydevice 100. The display device 100 described in this embodiment as anexample is a light-emitting display device in which a light-emittingelement is used as a display element. The display device 100 describedin this embodiment as an example has flexibility and can be bent freelyas illustrated in FIGS. 1B and 1C. FIG. 2 is a cross-sectional view of aportion taken along a dashed-dotted line A1-A2 in FIG. 1A.

<Structure of Display Device>

The display device 100 described in this embodiment as an exampleincludes a display area 131, a first driver circuit 132, and a seconddriver circuit 133. The display area 131. the first driver circuit 132,and the second driver circuit 133 include a plurality of transistors.For example, the second driver circuit 133 includes a plurality oftransistors 233. The display device 100 also includes a terminalelectrode 116 and a light-emitting element 125 including an electrode115, an EL layer 117, and an electrode 118. A plurality oflight-emitting elements 125 are formed in the display area 131. Atransistor 231 (not illustrated) for controlling the amount of lightemitted from the light-emitting element 125 is electrically connected toeach of the light-emitting elements 125. A capacitor 232 is electricallyconnected to the transistor 231. In addition, a transistor 431 capableof supplying a data signal is electrically connected to the transistor231.

The first driver circuit 132 and the second driver circuit 133 each havea function of supplying a signal from an external electrode 124 to aspecific one of the light-emitting elements 125 in the display area 131at a specific timing.

In the display device 100, a substrate 111 and a substrate 121 areattached to each other with a bonding layer 120 provided therebetween.An insulating film 205 is formed over the substrate 111 with a bondinglayer 112 provided therebetween. The insulating film 205 is preferablyformed as a single layer or a multilayer using silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, or the like. The insulatingfilm 205 can be formed by a sputtering method, a CVD method, a thermaloxidation method, a coating method, a printing method, or the like.

Note that the insulating film 205 functions as a base layer and canprevent or reduce diffusion of impurity elements from the substrate 111,the bonding layer 112, or the like to the transistor or thelight-emitting element.

An organic resin material, a glass material that is thin enough to haveflexibility, or the like can be used for the substrate 111 and thesubstrate 121. In the case where the display device 100 is a so-calledbottom-emission display device or a dual-emission display device, amaterial that transmits light emitted from the EL layer 117 is used forthe substrate 111. In the case where the display device 100 is atop-emission display device or a dual-emission display device, amaterial that transmits light emitted from the EL layer 117 is used forthe substrate 121.

Examples of materials that have flexibility and transmit visible light,which can be used for the substrate 111 and the substrate 121, include apolyethylene terephthalate resin, a polyethylene naphthalate resin, apolyacrylonitrile resin, a polyimide resin, a polymethylmethacrylateresin, a polycarbonate resin, a polyethersulfone resin, a polyamideresin, a cycloolefin resin, a polystyrene resin, a polyamide imideresin, a polyvinylchloride resin, or the like. Examples of substratesthat do not transmit light include a stainless steel substrate, asubstrate with stainless steel foil, a tungsten substrate, a substratewith tungsten foil, and the like.

Alternatively, polypropylene, polyester, polyvinyl fluoride, polyvinylchloride, polyamide, polyimide, an inorganic film formed by evaporation,paper, or the like can be used as the substrate 111 and the substrate121. Still alternatively, a cellophane substrate, a stone substrate, awood substrate, a cloth substrate (a natural fiber (e.g., silk, cotton,or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, a rubber substrate, orthe like can be used.

The thermal expansion coefficients of the substrate 111 and thesubstrate 121 are preferably less than or equal to 30 ppm/K, morepreferably less than or equal to 10 ppm/K. In addition, on surfaces ofthe substrate 111 and the substrate 121, a protective film having lowwater permeability may be formed in advance; examples of the protectivefilm include a film containing nitrogen and silicon such as a siliconnitride film or a silicon oxynitride film and a film containing nitrogenand aluminum such as an aluminum nitride film. Note that a structure inwhich a fibrous body is impregnated with an organic resin (also calledprepreg) may be used as the substrate 111 and the substrate 121.

With such substrates, a non-breakable display device can be provided.Alternatively, a lightweight display device can be provided.Alternatively, an easily bendable display device can be provided.

The transistor 231, the transistor 431, the capacitor 232, thetransistor 233, and the terminal electrode 116 are formed over theinsulating film 205 (see FIG. 2). Note that dual-gate transistors ineach of which a semiconductor layer where a channel is formed isprovided between two gate electrodes are described as examples of thetransistor 431 and the transistor 233 in this embodiment. However, thetransistor 431 and the transistor 233 can be single-gate transistors.For example, channel-protective transistors, top-gate transistors, orthe like can be used as the transistor 431 and the transistor 233.

The transistor 231 which is not illustrated in FIG. 2 can also have astructure similar to those of other transistors. The transistor 231, thetransistor 431, and the transistor 233 may have the same structure ordifferent structures. Nate that the size (e.g., channel length andchannel width) or the like of each transistor may be adjusted asappropriate.

The transistor 431 and the transistor 233 each include a gate electrode206, a gate insulating film 207, an oxide semiconductor film 208, asource electrode 209 a, and a drain electrode 209 b.

In addition, an insulating film 108 is formed over the transistor 431and the transistor 233, an insulating film 109 is formed over theinsulating film 108, and an insulating film 110 is formed over theinsulating film 109. The insulating film 110 functions as a protectiveinsulating layer and can prevent or reduce diffusion of impurityelements from a layer above the insulating film 110 to the transistor431 and the transistor 233.

Part of the insulating film 108 and the insulating film 109 is removedfrom a region not overlapping with the transistor 431 and the transistor233. The removal of the part of the insulating film 108 and theinsulating film 109 makes the display device 100 more easily bendable.

An insulating film 211 is formed over the insulating film 110.Planarization treatment may be performed on the insulating film 211 toreduce unevenness of a surface on which the light-emitting element 125is formed. The planarization treatment may be, but not particularlylimited to, polishing treatment (e.g., chemical mechanical polishing(CMP)) or dry etching treatment.

In addition, over the insulating film 211, the light-emitting element125 and a partition 114 for separating the light-emitting element 125from adjacent light-emitting elements 125 are formed.

In addition, the substrate 121 is provided with a light-blocking film264, a coloring layer 266, and an overcoat layer 268. The display device100 is a so-called top-emission display device, in which light 235emitted from the EL layer 117 is extracted from the substrate 121 sidethrough the coloring layer 266.

The light-emitting element 125 is electrically connected to thetransistor 231 via a wiring 241 through an opening formed in theinsulating film 211, the insulating film 110, the insulating film 109,and the insulating film 108.

In an opening overlapping with the terminal electrode 116 and providedin the insulating film 211, the insulating film 110, the insulating film109, and the insulating film 108, the external electrode 124 and theterminal electrode 116 are electrically connected to each other throughan anisotropic conductive connection layer 123. For example, an FPC canbe used as the external electrode 124.

The anisotropic conductive connection layer 123 can be formed using anyof known anisotropic conductive films (ACF), anisotropic conductivepastes (ACP), and the like.

The anisotropic conductive connection layer 123 is formed by curing apaste-form or sheet-form material that is obtained by mixing conductiveparticles to a thermosetting resin or a thermosetting, light-curableresin. The anisotropic conductive connection layer 123 exhibits ananisotropic conductive property by light irradiation orthermocompression bonding. As the conductive particles used for theanisotropic conductive connection layer 123, for example, particles of aspherical organic resin coated with a thin-film metal such as Au, Ni, orCo can be used.

By electrical connection between the external electrode 124 and theterminal electrode 116 through the anisotropic conductive connectionlayer 123, electric power or signals can be input to the display device100.

Note that a touch sensor may be formed over the substrate 121 asillustrated in FIG. 21. As the touch sensor, any of various types suchas a resistive type, a capacitive type, and an optical type can be used.As illustrated in FIG. 21, a touch sensor electrode 910 a and a touchsensor electrode 910 b are connected to each other through a wiring 912.In order to prevent contact with a touch sensor electrode 910 c, aninsulating layer 911 is provided thereover. The touch sensor electrodes910 a, 910 b, and 910 c are preferably formed using a transparentconductive film containing indium tin oxide, indium zinc oxide, or thelike so as to be able to transmit light. The wiring 912 is provided in asmall area and therefore can be formed using a single-layer film or amultilayer film containing a non-light-transmitting conductive materialsuch as Al, Mo, Ti, or W. Note that a transparent conductive filmcontaining indium tin oxide or indium zinc oxide may be used. The touchsensor directly formed over the substrate 121 as illustrated in FIG. 21has the advantage of not being displaced easily when the display device100 is bent.

Note that an optical sheet such as a polarizing plate or a retardationplate may be provided over the substrate 121.

Note that a touch sensor may be provided over a substrate other than thesubstrate 121. FIG. 22 illustrates an example in which a touch sensor isprovided over a substrate other than the substrate 121. For example, asubstrate 921 is an outermost substrate and corresponds to a cover forthe display device 100. Thus, the display device 100 is operated bydirectly touching the cover with a human finger or a stylus pen. In theexample illustrated in FIG. 22, the touch sensor is provided on a backside of the substrate 921. A bonding layer 920 is provided between thesubstrate 921 and the substrate 121 to fix these substrates. The bondinglayer 920 may be formed using a material similar to that of the bondinglayer 120. This has the advantage of not easily causing displacementeven when the display device 100 is bent. In addition, there is nointermediate layer of air, and therefore, external light is notreflected easily. Thus, an increase in visibility is another advantage.

Note that the substrate 921 is preferably formed using a materialsimilar to that of the substrates 121 and 111 when the display device100 is used in a bent state. However, the display device 100 isnon-breakable, and when the display device 100 is not used in a bentstate, the substrate 921 may be a glass substrate. In particular, withthe use of chemically-treated reinforced glass, a display device that isnot easily scratched and is durable can be provided. For example, glassmade of alkali alumino silicate can be used. The display device 100 isbendable and is therefore unlikely to be broken even when dropped; thus,a durable display device can be provided.

<Example of Pixel Circuit Configuration>

Next, an example of a specific configuration of the display device 100is described with reference to FIGS. 3A to 3C. FIG. 3A is a blockdiagram illustrating the configuration of the display device 100. Thedisplay device 100 includes the display area 131, the first drivercircuit 132, and the second driver circuit 133. The first driver circuit132 functions as a scan line driver circuit, for example, and the seconddriver circuit 133 functions as a signal line driver circuit, forexample.

The display device 100 includes m scan lines 135 which are arranged inparallel or substantially in parallel to each other and whose potentialsare controlled by the first driver circuit 132, and n signal lines 136which are arranged in parallel or substantially in parallel to eachother and whose potentials are controlled by the second driver circuit133. The display area 131 includes a plurality of pixels 134 arranged ina matrix. The first driver circuit 132 and the second driver circuit 133are collectively referred to as a driver circuit portion in some cases.

Each of the scan lines 135 is electrically connected to the n pixels 134in the corresponding row among the pixels 134 arranged in m rows and ncolumns in the display area 131. Each of the signal lines 136 iselectrically connected to the m pixels 134 in the corresponding columnamong the pixels 134 arranged in m rows and n columns. Note that m and nare each an integer of 1 or more.

FIGS. 3B and 3C illustrate circuit configurations that can be used forthe pixels 134 in the display device illustrated in FIG. 3A.

[Example of Pixel Circuit for Light-Emitting Display Device]

The pixel 134 illustrated in FIG. 3B includes a transistor 431, thecapacitor 232, the transistor 231, and the light-emitting element 125.

One of a source electrode and a drain electrode of the transistor 431 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 431 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 431 has a function of controlling whether to write a datasignal to a node 435 by being turned on or off.

One of a pair of electrodes of the capacitor 232 is electricallyconnected to a wiring to which a particular potential is supplied(hereinafter referred to as a potential supply line VL_a), and the otheris electrically connected to the node 435. The other of the sourceelectrode and the drain electrode of the transistor 431 is electricallyconnected to the node 435.

The capacitor 232 functions as a storage capacitor for storing datawritten to the node 435.

One of a source electrode and a drain electrode of the transistor 231 iselectrically connected to the potential supply line VL_a. A gateelectrode of the transistor 231 is electrically connected to the node435.

One of an anode and a cathode of the light-emitting element 125 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 231.

As the light-emitting element 125, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 125 is not limited toorganic EL elements; an inorganic EL element including an inorganicmaterial can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel 134 in FIG. 39, the pixels 134are sequentially selected row by row by the first driver circuit 132,whereby the transistors 431 are turned on and a data signal is writtento the nodes 435.

When the transistors 431 are turned off, the pixels 134 in which thedata has been written to the nodes 435 are brought into a holding state.Further, the amount of current flowing between the source electrode andthe drain electrode of the transistor 231 is controlled in accordancewith the potential of the data written to the node 435. Thelight-emitting element 125 emits light with a luminance corresponding tothe amount of flowing current. This operation is sequentially performedrow by row; thus, an image is displayed.

[Example of Pixel Circuit for Liquid Crystal Display Device]

The pixel 134 illustrated in FIG. 3C includes a liquid crystal element432, the transistor 431, and the capacitor 232.

The potential of one of a pair of electrodes of the liquid crystalelement 432 is set according to the specifications of the pixels 134 asappropriate. The alignment state of the liquid crystal element 432depends on data written to a node 436. A common potential may be appliedto one of the pair of electrodes of the liquid crystal element 432included in each of the plurality of pixels 134. Further, the potentialsupplied to one of a pair of electrodes of the liquid crystal element432 in the pixel 134 in one row may be different from the potentialsupplied to one of a pair of electrodes of the liquid crystal element432 in the pixel 134 in another row.

As examples of a driving method of the display device including theliquid crystal element 432, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optically compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude an electrically controlled birefringence (ECB) mode, a polymerdispersed liquid crystal (PDLC) mode, a polymer network liquid crystal(PNLC) mode, and a guest-host mode. Note that the present invention isnot limited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

The liquid crystal element 432 may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

Note that a display element other than the light-emitting element 125and the liquid crystal element 432 can be used. For example, anelectrophoretic element, an electronic ink, an electrowetting element, amicro electro mechanical system (MEMS), a digital micromirror device(DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator (IMOD) element, or the like can be used as thedisplay element.

In the pixel 134 in the m-th row and the n-th column, one of a sourceelectrode and a drain electrode of the transistor 431 is electricallyconnected to a signal line DL_n, and the other is electrically connectedto the node 436. A gate electrode of the transistor 431 is electricallyconnected to a scan line GL_m. The transistor 431 has a function ofcontrolling whether to write a data signal to the node 436 by beingturned on or off.

One of a pair of electrodes of the capacitor 232 is electricallyconnected to a wiring to which a particular potential is supplied(hereinafter referred to as a capacitor line CL), and the other iselectrically connected to the node 436. The other of the pair ofelectrodes of the liquid crystal element 432 is electrically connectedto the node 436. The potential of the capacitor line CL is set inaccordance with the specifications of the pixel 134 as appropriate. Thecapacitor 232 functions as a storage capacitor for storing data writtento the node 436.

For example, in the display device including the pixel 134 in FIG. 3C,the pixels 134 are sequentially selected row by row by the first drivercircuit 132, whereby the transistors 431 are turned on and a data signalis written to the nodes 436.

When the transistors 431 are turned off, the pixels 134 in which thedata signal has been written to the nodes 436 are brought into a holdingstate. This operation is sequentially performed row by row thus, animage is displayed.

<Example of Manufacturing Method>

Next, an example of a method for manufacturing the display device 100will be described with reference to cross-sectional views in FIGS. 4A to4C, FIGS. 5A to 5C, FIGS. 6A and 6B, FIGS. 7A and 7B, FIGS. 8A and 8B,FIGS. 9 to 11, and FIGS. 12A to 12C. FIGS. 4A to 12C correspond to across-section of the display area 131 in FIG. 2.

[Formation of Separation Layer]

First, a separation layer 113 is formed over an element formationsubstrate 101 (see FIG. 4A). Note that the element formation substrate101 may be a glass substrate, a quartz substrate, a sapphire substrate,a ceramic substrate, a metal substrate, or the like. Alternatively, theelement formation substrate 101 may be a plastic substrate having heatresistance to the processing temperature in this embodiment.

As the glass substrate, for example, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used. Note that when the glass substrate contains a largeamount of barium oxide (BaO), the glass substrate can be heat-resistantand more practical. Alternatively, crystallized glass or the like may beused.

The separation layer 113 can be formed using an element selected fromtungsten, molybdenum, titanium, tantalum, niobium, nickel, cobalt,zirconium, ruthenium, rhodium, palladium, osmium, iridium, and silicon;an alloy material containing any of the elements; or a compound materialcontaining any of the elements. The separation layer 113 can also beformed to have a single-layer structure or a stacked-layer structureusing any of the materials. Note that the crystalline structure of theseparation layer 113 may be amorphous, microcrystalline, orpolycrystalline. The separation layer 113 can also be formed using ametal oxide such as aluminum oxide, gallium oxide, zinc oxide, titaniumdioxide, indium oxide, indium tin oxide, indium zinc oxide, or InGaZnO(IGZO).

The separation layer 113 can be formed by a sputtering method, a CVDmethod, a coating method, a printing method, or the like. Note that thecoating method includes a spin coating method, a droplet dischargemethod, and a dispensing method.

In the case where the separation layer 113 has a single-layer structure,the separation layer 113 is preferably formed using tungsten,molybdenum, or a tungsten-molybdenum alloy. Alternatively, theseparation layer 113 is preferably formed using an oxide or oxynitrideof tungsten, an oxide or oxynitride of molybdenum, or an oxide oroxynitride of a tungsten-molybdenum alloy.

In the case where the separation layer 113 has a stacked-layer structureincluding, for example, a layer containing tungsten and a layercontaining an oxide of tungsten, the layer containing an oxide oftungsten may be formed as follows: the layer containing tungsten isformed first and then an oxide insulating film is formed in contacttherewith, so that the layer containing an oxide of tungsten is formedat the interface between the layer containing tungsten and the oxideinsulating film. Alternatively, the layer containing an oxide oftungsten may be formed by performing thermal oxidation treatment, oxygenplasma treatment, treatment with a highly oxidizing solution such asozone water, or the like on the surface of the layer containingtungsten.

In this embodiment, the separation layer 113 is formed of tungsten by asputtering method.

[Formation of Base Layer]

Next, the insulating film 205 is formed as a base layer over theseparation layer 113 (see FIG. 4A). The insulating film 205 ispreferably formed as a single layer or a multilayer using silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, aluminum nitride oxide, or the like. Theinsulating film 205 can be formed by a sputtering method, a CVD method,a thermal oxidation method, a coating method, a printing method, or thelike.

The thickness of the insulating film 205 may be greater than or equal to30 nm and less than or equal to 500 nm, preferably greater than or equalto 50 nm and less than or equal to 400 nm.

The insulating film 205 can prevent or reduce diffusion of impurityelements from the substrate 111, the bonding layer 112, or the like tothe light-emitting element 125. In this embodiment, a silicon oxide filmhaving a thickness of 200 nm is formed as the insulating film 205 by aplasma CVD method.

[Formation of Gate Electrode]

Next, the gate electrode 206 is formed over the insulating film 205 (seeFIG. 4A). The gate electrode 206 can be formed using a metal elementselected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing any of these metalelements as a component; an alloy containing any of these metal elementsin combination; or the like. Further, one or more metal elementsselected from manganese and zirconium may be used. The gate electrode206 may have a single-layer structure or a stacked structure of two ormore layers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a two-layer structure in whicha copper film is stacked over a titanium film, a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order, and the like can be given. Alternatively, a film,an alloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 206 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

First, a conductive film to be the gate electrode 206 is stacked overthe insulating film 205 by a sputtering method, a CVD method, anevaporation method, or the like, and a resist mask is formed over theconductive film by a photolithography process. Next, part of theconductive film to be the gate electrode 206 is etched with the use ofthe resist mask to form the gate electrode 206. At the same time, awiring and another electrode can be formed.

The conductive film may be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Note thatin the case where the conductive film is etched by a dry etching method,asking treatment may be performed before the resist mask is removed,whereby the resist mask can be easily removed using a stripper.

Note that the gate electrode 206 may be formed by an electrolyticplating method, a printing method, an inkjet method, or the like insteadof the above formation method.

The thickness of the gate electrode 206 is greater than or equal to 5 nmand less than or equal to 500 nm, preferably greater than or equal to 10nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm.

The gate electrode 206 may be formed using a light-blocking conductivematerial, whereby external light can be prevented from reaching theoxide semiconductor film 208 from the gate electrode 206 side. As aresult, a variation in electrical characteristics of the transistor dueto light irradiation can be suppressed.

[Formation of Gate Insulating Film]

Next, the gate insulating film 207 is formed (see FIG. 4A). The gateinsulating film 207 can be formed to have a single-layer structure or astacked-layer structure using, for example, any of silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, a mixture of aluminum oxide and silicon oxide, hafnium oxide,gallium oxide, Ga—Zn-based metal oxide, and the like.

The gate insulating film 207 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced. For example, a stackedlayer of silicon oxynitride and hafnium oxide may be used.

The thickness of the gate insulating film 207 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The gate insulating film 207 can be formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the gate insulating film 207, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples.

The gate insulating film 207 can have a stacked-layer structure in whicha nitride insulating film and an oxide insulating film are stacked inthat order from the gate electrode 206 side. When the nitride insulatingfilm is provided on the gate electrode 206 side, an impurity, typicallyhydrogen, nitrogen, alkali metal, alkaline earth metal, or the like, canbe prevented from moving from the gate electrode 206 side to the oxidesemiconductor film 208. Further, when the oxide insulating film isprovided on the oxide semiconductor film 208 side, the density of defectstates at the interface between the gate insulating film 207 and theoxide semiconductor film 208 can be reduced. Consequently, a transistorwhose electrical characteristics are hardly degraded can be obtained.Note that it is preferable to form, as the oxide insulating film, anoxide insulating film containing oxygen at a higher proportion than thestoichiometric composition. This is because the density of defect statesat the interface between the gate insulating film 207 and the oxidesemiconductor film 208 can be further reduced.

In the case where the gate insulating film 207 is a stacked layer of anitride insulating film and an oxide insulating film as described above,it is preferable that the nitride insulating film be thicker than theoxide insulating film.

The nitride insulating film has a dielectric constant higher than thatof the oxide insulating film; therefore, an electric field generatedfrom the gate electrode 206 can be efficiently transmitted to the oxidesemiconductor film 208 even when the gate insulating film 207 has alarge thickness. When the gate insulating film 207 has a large totalthickness, the withstand voltage of the gate insulating film 207 can beincreased. Accordingly, the reliability of the semiconductor device canbe improved.

The gate insulating film 207 can have a stacked-layer structure in whicha first nitride insulating film with few defects, a second nitrideinsulating film with a high blocking property against hydrogen, and anoxide insulating film are stacked in that order from the gate electrode206 side. When the first nitride insulating film with few defects isused in the gate insulating film 207, the withstand voltage of the gateinsulating film 207 can be improved. Further, when the second nitrideinsulating film with a high blocking property against hydrogen isprovided in the gate insulating film 207, hydrogen contained in the gateelectrode 206 and the first nitride insulating film can be preventedfrom moving to the oxide semiconductor film 208.

An example of a method for forming the first and second nitrideinsulating films is described below. First, a silicon nitride film withfew defects is formed as the first nitride insulating film by a plasmaCVD method in which a mixed gas of silane, nitrogen, and ammonia is usedas a source gas. Next, a silicon nitride film in which the hydrogenconcentration is low and hydrogen can be blocked is formed as the secondnitride insulating film by switching the source gas to a mixed gas ofsilane and nitrogen. By such a formation method, the gate insulating,film 207 in which nitride insulating films with few defects and ablocking property against hydrogen are stacked can he formed.

The gate insulating film 207 can have a stacked-layer structure in whicha third nitride insulating film with a high blocking property against animpurity, the first nitride insulating film with few defects, the secondnitride insulating film with a high blocking property against hydrogen,and the oxide insulating film are stacked in that order from the gateelectrode 206 side. When the third nitride insulating film with a highblocking property against an impurity is provided in the gate insulatingfilm 207, an impurity, typically hydrogen, nitrogen, alkali metal,alkaline earth metal, or the like, can be prevented from moving from thegate electrode 206 to the oxide semiconductor film 208.

An example of a method for forming the first to third nitride insulatingfilms is described below. First, a silicon nitride film with a highblocking property against an impurity is formed as the third nitrideinsulating film by a plasma CVD method in which a mixed gas of silane,nitrogen, and ammonia is used as a source gas. Next, a silicon nitridefilm with few defects is formed as the first nitride insulating film byincreasing the flow rate of ammonia. Then, a silicon nitride film inwhich the hydrogen concentration is low and hydrogen can be blocked isformed as the second nitride insulating film by switching the source gasto a mixed gas of silane and nitrogen. By such a formation method, thegate insulating film 207 in which nitride insulating films with fewdefects and a blocking property against an impurity are stacked can beformed.

Moreover, in the case of forming a gallium oxide film as the gateinsulating film 207, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Note that the threshold voltage of a transistor can be changed bystacking the oxide semiconductor film 208 in which a channel of thetransistor is formed and an insulating film containing hafnium oxidewith an oxide insulating film provided therebetween and injectingelectrons into the insulating film containing hafnium oxide.

[Formation of Oxide Semiconductor Film]

Next, the oxide semiconductor film 208 in which a channel is formed andan oxide semiconductor film 209 serving as one electrode of thecapacitor 232 later are formed over the gate insulating film 207 (seeFIG. 4B). Typical examples of a material that can be used for the oxidesemiconductor film 208 and the oxide semiconductor film 209 include anIn—Ga oxide, an In—Zn oxide, and an In—M—Zn oxide (M represents Al, Ga,Y, Zr, La, Ce, or Nd).

In the case where the oxide semiconductor film 208 contains an In—M—Znoxide, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is greater than or equal to 25 atomic % and the atomic percentage ofM is less than 75 atomic %, or more preferably, the atomic percentage ofIn is greater than or equal to 34 atomic % and the atomic percentage ofM is less than 66 atomic %.

The energy gap of the oxide semiconductor film 208 is particularly 2 eVor more, preferably 2.5 eV or more, more preferably 3 eV or more. Withthe use of an oxide semiconductor having such a wide energy gap, theoff-state current of the transistor can be reduced.

The thickness of the oxide semiconductor film 208 and the oxidesemiconductor film 209 is set to greater than or equal to 3 nm and lessthan or equal to 200 nm, preferably greater than or equal to 3 nm andless than or equal to 100 nm, more preferably greater than or equal to 3nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 208 and the oxidesemiconductor film 209 contain an In—M—Zn oxide (M represents Al, Ga, Y,Zr, La, Ce, or Nd), the atomic ratio of metal elements of a sputteringtarget used for depositing the In—M—Zn oxide preferably satisfies In≥Mand Zn>M. As the atomic ratio of metal elements of such a sputteringtarget, In:M:Zn=1:1:1, In:M:Zn=5:5:6, In:M:Zn=2:1:2, and In:M:Zn=3:1:2are preferable. Note that the atomic ratios of metal elements in theoxide semiconductor film 208 and the oxide semiconductor film 209 varyfrom those in the above-described sputtering target, within a range of±40% as an error. When the content of In in the oxide semiconductor film208 is high, the on-state current and the field-affect mobility of thetransistor are increased. Thus, when the oxide semiconductor film 208 isformed using a sputtering target of the In—M—Zn oxide having an atomicratio of In:M:Zn=3:1:2, the transistor having excellent electricalcharacteristics can be fabricated.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 208. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ orlower, more preferably 1×10¹³/cm³ or lower, still more preferably1×10¹¹/cm³ or lower is used as the oxide semiconductor film 208.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. Further,in order to obtain required semiconductor characteristics of atransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 208 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 208,an oxide semiconductor film in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. The state in whichthe impurity concentration is low and the density of defect states islow (the number of oxygen vacancies is small) is referred to as a“highly purified intrinsic” or “substantially highly purified intrinsic”state. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor has few carrier generation sources, andthus can have a low carrier density in some cases. Thus, a transistor inwhich a channel region is formed in the oxide semiconductor film rarelyhas a negative threshold voltage (is rarely normally on). Further, ahighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states and thus hasa low density of trap states in some cases.

Further, a transistor including a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film as asemiconductor film in which a channel is formed has an extremely lowoff-state current; even when the transistor has a channel width of 1×10⁶μm and a channel length (L) of 10 μm, the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of from 1 V to10 V. Thus, the transistor whose channel region is formed in the oxidesemiconductor film has a small variation in electrical characteristicsand high reliability in some cases. Electric charges trapped by the trapstates in the oxide semiconductor film take a long time to be lost, andmight behave like fixed electric charges. Thus, the transistor in whicha channel region is formed in the oxide semiconductor film having a highdensity of trap states has unstable electrical characteristics in somecases. Examples of the impurities include hydrogen, nitrogen, alkalimetal, alkaline earth metal, and the like.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to form water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or in a portionfrom which oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible in the oxide semiconductor film 208. Specifically, the hydrogenconcentration of the oxide semiconductor film 208, which is measured bysecondary ion mass spectrometry (SIMS), is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, morepreferably lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁸ atoms/cm³, yet more preferablylower than or equal to 1×10¹⁸ atoms/cm³, even more preferably lower thanor equal to 5×10¹⁷ atoms/cm³, or further preferably lower than or equalto 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 208, oxygen vacancies areincreased, and the oxide semiconductor film 208 becomes n-type. Thus,the concentration of silicon or carbon (the concentration is measured bySIMS) of the oxide semiconductor film 208 is lower than or equal to2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 208, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 208.

Further, when containing nitrogen, the oxide semiconductor film 208easily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

A structure of an oxide semiconductor film is described below.

An oxide semiconductor film is classified roughly into anon-single-crystal oxide semiconductor film and a single crystal oxidesemiconductor film. The non-single-crystal oxide semiconductor filmincludes any of a c-axis aligned crystalline oxide semiconductor(CAAC-OS) film, a polycrystalline oxide semiconductor film, amicrocrystalline oxide semiconductor film, an amorphous oxidesemiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films including aplurality of c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface where theCAAC-OS film is formed (hereinafter, a surface where the CAAC-OS film isformed is also referred to as a formation surface or a top surface ofthe CAAC-OS film, and is arranged in parallel to the formation surfaceor the top surface of the CAAC-OS film.

On the other hand, according to a TEM image of the CAAC-OS film observedin a direction substantially perpendicular to the sample surface (planTEM image), metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case were acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (ϕ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (ϕaxis) with 2θ fixed at around 56°. In the case where the sample is asingle crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when ϕ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface of the CAAC-OS film. Thus, for example, in thecase where a shape of the CAAC-OS film is changed by etching or thelike, the c-axis might not be necessarily parallel to a normal vector ofa formation surface or a normal vector of a top surface of the CAAC-OSfilm.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in sonic cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film with a low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which the impurity concentration is low and the density ofdefect states is low (the number of oxygen vacancies is small) isreferred to as a “highly purified intrinsic” or “substantially highlypurified intrinsic” state. A highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has few carriergeneration sources, and thus can have a low carrier density. Thus, atransistor including the oxide semiconductor film rarely has negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasfew carrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand has high reliability. Electric charges trapped by the carrier trapsin the oxide semiconductor film take a long time to be released, andmight behave like fixed electric charges. Thus, the transistor whichincludes the oxide semiconductor film having high impurity concentrationand a high density of defect states has unstable electricalcharacteristics in some cases.

With use of the CAAC-OS film in a transistor, a variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a polycrystalline oxide semiconductor filth is described.

In an image of a polycrystalline oxide semiconductor film which isobtained with a TEM, crystal grains can be found. In most cases, thesize of the crystal grains in the polycrystalline oxide semiconductorfilm is greater than or equal to 2 nm and less than or equal to 300 nm,greater than or equal to 3 nm and less than or equal to 100 nm, orgreater than or equal to 5 nm and less than or equal to 50 nm in animage obtained with the TEM, for example. Moreover, in an image of thepolycrystalline oxide semiconductor film which is obtained with the TEM,a boundary between crystal grains can be found in some cases.

The polycrystalline oxide semiconductor film may include a plurality ofcrystal grains, and the plurality of crystal grains may be oriented indifferent directions. A polycrystalline oxide semiconductor film issubjected to structural analysis with an XRD apparatus. For example,when a polycrystalline oxide semiconductor film including an InGaZnO₄crystal is analyzed by an out-of-plane method, peaks of 2θ appear ataround 31°, 36°, and the like in some cases.

The polycrystalline oxide semiconductor film has high crystallinity andthus has high electron mobility in some cases. Accordingly, a transistorincluding the polycrystalline oxide semiconductor film has highfield-effect mobility. Note that there are cases in which an impurity issegregated at the grain boundary in the polycrystalline oxidesemiconductor film. Moreover, the grain boundary of the polycrystallineoxide semiconductor film becomes a defect state. Since the grainboundary of the polycrystalline oxide semiconductor film may serve as acarrier trap or a carrier generation source, the transistor includingthe polycrystalline oxide semiconductor film has larger variation inelectrical characteristics and lower reliability than a transistorincluding a CAAC-OS film in some cases.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor film in some cases. In mostcases, the size of a crystal part in the microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. A microcrystal with a size greater than or equal to 1 nm andless than or equal to 10 nm, or a size greater than or equal to 1 nm andless than or equal to 3 nm, is specifically referred to as nanocrystal(nc). An oxide semiconductor film including nanocrystal is referred toas an nc-OS (nanocrystalline oxide semiconductor) film. In an image ofthe nc-OS film which is obtained with the TEM, for example, a boundaryis not clearly detected in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the size of a crystal part, a peak which shows acrystal plane does not appear. Further, a diffraction pattern like ahalo pattern appears in a selected-area electron diffraction pattern ofthe nc-OS film which is obtained by using an electron beam having adiameter (e.g., larger than or equal to 50 nm) larger than the size of acrystal part. Meanwhile, spots are observed in a nanobeam electrondiffraction pattern of the nc-OS film obtained by using an electron beamhaving a diameter e.g., larger than or equal to 1 nm and smaller than orequal to 30 nm) close to or smaller than the size of a crystal part.Further, in a nanobeam electron diffraction pattern of the nc-OS film,for example, bright regions in a circular (or ring-shaped) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS film, a plurality of spots are shown in a ring-like region insome cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

Accordingly, the nc-OS film has higher carrier density than the CAAC-OSfilm in some cases. An oxide semiconductor film with a high carrierdensity tends to have a high electron mobility. Therefore, a transistorincluding the nc-OS film has a high field-effect mobility in some cases.The nc-OS film has a higher density of defect states than the CAAC-OSfilm, and thus may have a lot of carrier traps. Consequently, thetransistor including the nc-OS film has larger variation in electricalcharacteristics and lower reliability than a transistor including theCAAC-OS film. Note that the nc-OS film can be easily formed as comparedto the CAAC-OS film because the nc-OS film can be obtained even when theamount of impurity contained therein is relatively large; thus, thenc-OS film is sometimes preferably used depending on the application.Therefore, a semiconductor device including the transistor including thenc-OS film can be manufactured with high productivity.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal parts. For example, the amorphous oxide semiconductorfilm does not have a specific state as in quartz.

In an image obtained with the TEM, crystal parts cannot be found in theamorphous oxide semiconductor film.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed in anelectron diffraction pattern of the amorphous oxide semiconductor film.Further, a halo pattern is observed but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

The amorphous oxide semiconductor film contains impurities such ashydrogen at a high concentration. In addition, the amorphous oxidesemiconductor film has a high density of defect states.

The oxide semiconductor film having a high impurity concentration and ahigh density of detect states has many carrier traps or many carriergeneration sources.

Thus, the amorphous oxide semiconductor film has a much higher carrierdensity than the nc-OS film in some cases. Therefore, a transistorincluding the amorphous oxide semiconductor film tends to be normallyon. Therefore, in some cases, such an amorphous oxide semiconductor filmcan be applied to a transistor which needs to be normally on. Since theamorphous oxide semiconductor film has a high density of defect states,carrier traps might be increased. Consequently, a transistor includingthe amorphous oxide semiconductor film has larger variation inelectrical characteristics and lower reliability than a transistorincluding the CAAC-OS film or the nc-OS film.

Next, a single crystal oxide semiconductor film is described.

The single crystal oxide semiconductor film has a lower impurityconcentration and a lower density of defect states (few oxygenvacancies). Thus, the carrier density can be decreased. Therefore, atransistor including the single crystal oxide semiconductor film isunlikely to be normally on. Moreover, since the single crystal oxidesemiconductor film has a lower impurity concentration and a lowerdensity of defect states, carrier traps might be reduced. Therefore, atransistor including the single crystal oxide semiconductor film has asmall variation in electrical characteristics and a high reliability insome cases.

Note that when the oxide semiconductor film has few defects, the densitythereof is increased. When the oxide semiconductor film has highcrystallinity, the density thereof is increased. When the oxidesemiconductor film has a lower concentration of impurities such ashydrogen, the density thereof is increased. The single crystal oxidesemiconductor film has a higher density than the CAAC-OS film. TheCAAC-OS film has a higher density than the microcrystalline oxidesemiconductor film. The polycrystalline oxide semiconductor film has ahigher density than the microcrystalline oxide semiconductor film. Themicrocrystalline oxide semiconductor film has a higher density than theamorphous oxide semiconductor film.

Note that an oxide semiconductor film may be a stacked film includingtwo or more kinds of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

A method for forming the oxide semiconductor film 208 and the oxidesemiconductor film 209 is described below. An oxide semiconductor filmwhich is to be the oxide semiconductor film 208 and the oxidesemiconductor film 209 is formed over the gate insulating film 207.Then, after a resist mask is formed over the oxide semiconductor film bya photolithography process, part of the oxide semiconductor film isetched using the resist mask. Thus, the oxide semiconductor film 208 andthe oxide semiconductor film 209 can be formed.

The oxide semiconductor film can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, a CVD method, or the like. Note that in the case where an oxidesemiconductor film is formed by a sputtering method, a power supplydevice for generating plasma can be an RF power supply device, an ACpower supply device, a DC power supply device, or the like asappropriate.

As a sputtering gas, a rare gas (typically argon) atmosphere, an oxygenatmosphere, or a mixed gas of a rare gas and oxygen is used asappropriate. In the case of using the mixed gas of a rare gas andoxygen, the proportion of oxygen to the rare gas is preferablyincreased.

Further, a sputtering target may be appropriately selected in accordancewith the composition of the oxide semiconductor film to be formed.

In order to obtain a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film, it is necessary to highlypurify a sputtering gas as well as to evacuate a chamber to a highvacuum. As an oxygen gas or an argon gas used for a sputtering gas, agas which is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, more preferably −100° C. or lower, or stillpreferably −120° C. or lower is used, whereby entry of moisture or thelike into the oxide semiconductor film can be minimized.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide targetwith an atomic ratio of In:Ga:Zn=1:1:1. Next, a resist mask is formedover the oxide semiconductor film, and part of the oxide semiconductorfilm is selectively etched. Thus, the oxide semiconductor film 208 andthe oxide semiconductor film 209 can be formed.

Then, first heat treatment may be performed. The first heat treatmentcan reduce the concentrations of hydrogen and water contained in theoxide semiconductor film 208 and the oxide semiconductor film 209 byreleasing hydrogen, water, and the like from the oxide semiconductorfilm 208 and the oxide semiconductor film 209. The heat treatment isperformed typically at a temperature of higher than or equal to 300° C.and lower than or equal to 400° C., preferably higher than or equal to320° C. and lower than or equal to 370° C.

The first heat treatment can be performed using an electric furnace, anRTA apparatus, or the like. With the use of an RTA apparatus, the heattreatment can be performed at a temperature of higher than or equal tothe strain point of the substrate if the heating time is short.Therefore, the heat treatment time can be shortened.

The first heat treatment may be performed under an atmosphere ofnitrogen, oxygen, ultra-dry air (air with a water content of 20 ppm orless, preferably 1 ppm or less, more preferably 10 ppb or less), or arare gas (argon, helium, or the like). The atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas preferably does not containhydrogen, water, and the like. Further, after heat treatment performedin a nitrogen atmosphere or a rare gas atmosphere, heat treatment may beadditionally performed in an oxygen atmosphere or an ultra-dry airatmosphere. As a result, hydrogen, water, and the like can be releasedfrom the oxide semiconductor film 208 and the oxide semiconductor film209 and oxygen can be supplied to the oxide semiconductor film 208 andthe oxide semiconductor film 209 at the same time. Consequently, theamount of oxygen vacancies in the oxide semiconductor film 208 and theoxide semiconductor film 209 can be reduced.

Although an example in which the oxide semiconductor film 209 is formedas one electrode of the capacitor 232 is described, one embodiment ofthe present invention is not limited to this example. In some cases ordepending on the situation, as one electrode of the capacitor 232, aconductive film which does not have a light-transmitting property may beused, or a film which is formed in a step different from that for theoxide semiconductor film 208 may be used. In other words, one electrodeof the capacitor 232 may be provided so as not to be in contact with anupper surface of the gate insulating film 207.

[Formation of Source Electrode and Drain Electrode]

Next, the source electrode 209 a, the drain electrode 209 b, and anelectrode 210 are formed. First, a conductive film 220 is formed overthe insulating film 205, the oxide semiconductor film 208, and the oxidesemiconductor film 209 (see FIG. 4C).

The conductive film 220 can have a single-layer structure or astacked-layer structure including any of metals such as aluminum,titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum,silver, tantalum, and tungsten or an alloy containing any of thesemetals as its main component. For example, a single-layer structure ofan aluminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, a three-layer structure in whicha molybdenum film or a molybdenum nitride film, an aluminum film or acopper film, and a molybdenum film or a molybdenum nitride film arestacked in this order, a three-layer structure in which a tungsten film,a copper film, and a tungsten film are stacked in this order, and thelike can be given.

Note that a conductive material containing oxygen such as indium tinoxide, zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, or a conductive materialcontaining nitrogen such as titanium nitride or tantalum nitride may beused. It is also possible to employ a stacked-layer structure formedusing a material containing the above metal element and conductivematerial containing oxygen. It is also possible to employ astacked-layer structure formed using a material containing the abovemetal element and conductive material containing nitrogen. It is alsopossible to use a stacked-layer structure formed using a materialcontaining the above metal element, conductive material containingoxygen, and conductive material containing nitrogen.

The thickness of the conductive film 220 is greater than or equal to 5nm and less than or equal to 500 nm, preferably greater than or equal to10 nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm. In this embodiment, a300-nm-thick tungsten film is formed as the conductive film 220.

Next, a resist mask is formed over the conductive film 220 by aphotolithography process, and part of the conductive film 220 isselectively etched using the resist mask. Thus, the source electrode 209a, the drain electrode 209 b, and the electrode 210 are formed. Anotherelectrode and a wiring, such as the terminal electrode 116, can beformed at the same time.

Note that the conductive film 220 may be etched by a dry etching method,a wet etching method, or both a dry etching method and a wet etchingmethod. Note that an exposed portion of the oxide semiconductor film maybe removed by the etching step in some cases see FIG. 5A).

[Formation of Oxide Insulating Film]

Next, the insulating film 108 is formed. The insulating film 108 is anoxide insulating film which is permeable to oxygen. Note that theinsulating film 108 serves also as a film which relieves damage to theoxide semiconductor film 208 at the time of forming the insulating film109 later (see FIG. 5B).

Silicon oxide, silicon oxynitride, or the like with a thickness greaterthan or equal to 5 nm and less than or equal to 150 nm, preferablygreater than or equal to 5 nm and less than or equal to 50 nm can beused as the insulating film 108. Note that in this specification, an“oxynitride film” refers to a film that contains oxygen at a higherproportion than nitrogen, and a “nitride oxide film” refers to a filmthat contains nitrogen at a higher proportion than oxygen.

Further, it is preferable that the amount of defects in the insulatingfilm 108 be small, and typically the spin density corresponding to anESR signal at g=2.001 due to a dangling bond of silicon be lower than orequal to 3×10¹⁷ spins/cm³. This is because if the density of defects inthe insulating film 108 is high, oxygen is bonded to the detects and theamount of oxygen that permeates the insulating film 108 is decreased.

Further, it is preferable that the amount of defects at the interfacebetween the insulating film 108 and the oxide semiconductor film 208 besmall, and typically the spin density corresponding to a signal whichappears at g=1.93 due to a defect in the oxide semiconductor film 208 belower than or equal to 1×10¹⁷ spins/cm³, more preferably lower than orequal to the lower limit of detection by ESR measurement.

Note that in the insulating film 108, all oxygen having entered theinsulating film 108 from the outside does not move to the outside of theinsulating film 108 and some oxygen remains in the insulating film 108.Further, movement of oxygen occurs in the insulating film 108 in somecases in such a manner that oxygen enters the insulating film 108 andoxygen contained in the insulating film 108 is moved to the outside ofthe insulating film 108.

When the oxide insulating film which is permeable to oxygen is formed asthe insulating film 108, oxygen released from the insulating film 109provided over the insulating film 108 can be moved to the oxidesemiconductor film 208 through the insulating film 108.

A silicon oxide film or a silicon oxynitride film is preferably used asthe insulating film 108. The silicon oxide film or the siliconoxynitride film used as the insulating film 108 can be formed under thefollowing conditions: the substrate placed in a treatment chamber of aplasma CVD apparatus that is vacuum-evacuated is held at a temperaturehigher than or equal to 280° C. and lower than or equal to 400° C., thepressure in the treatment chamber is greater than or equal to 20 Pa andless than or equal to 250 Pa, preferably greater than or equal to 100 Paand less than or equal to 250 Pa with introduction of a source gas intothe treatment chamber, and a high-frequency power is supplied to anelectrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as a source gas for forming the silicon oxide film or the siliconoxynitride film. Typical examples of the deposition gas containingsilicon include same, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples.

With the use of the above conditions, an oxide insulating film which ispermeable to oxygen can be formed as the insulating film 108. Further,by providing the insulating film 108, damage to the oxide semiconductorfilm 208 can be reduced in a step of forming the insulating film 109.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the insulating film 108, a dense and hard oxideinsulating film which is permeable to oxygen, typically, a silicon oxidefilm or a silicon oxynitride film of which etching using hydrofluoricacid of 0.5 wt % at 25° C. is performed at a rate of lower than or equalto 10 nm/min, preferably lower than or equal to 8 nm/min can be formed.

The insulating film 108 is formed while heating is performed; thus,hydrogen, water, or the like contained in the oxide semiconductor film208 and the oxide semiconductor film 209 can be released in the step.Specifically, the insulating film 108 is formed while the elementformation substrate 101 is kept at 280° C. to 400° C.; thus, hydrogen,water, or the like contained in the oxide semiconductor film 208 can bereleased. Hydrogen contained in the oxide semiconductor film 208 isbonded to an oxygen radical formed in plasma to form water. Since thesubstrate is heated in the step of forming the insulating film 108,water formed by bonding of oxygen and hydrogen is released from theoxide semiconductor film. That is, when the insulating film 108 isformed by a plasma CVD method, the amount of water and hydrogencontained in the oxide semiconductor film can be reduced.

Further, time for heating in a state where the oxide semiconductor film208 and the oxide semiconductor film 209 are exposed can be shortenedbecause healing is performed in the step of forming the insulating film108. Thus, the amount of oxygen released from the oxide semiconductorfilms by heat treatment can be reduced. That is, the amount of oxygenvacancies in the oxide semiconductor films can be reduced.

Furthermore, by setting the pressure in the treatment chamber to begreater than or equal to 100 Pa and less than or equal to 250 Pa, theamount of water contained in the insulating film 108 is reduced; thus,variation in electrical characteristics of the transistor can be reducedand change in threshold voltage can be inhibited.

Moreover, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, damage to theoxide semiconductor film 208 and the oxide semiconductor film 209 can bereduced when the insulating film 108 is formed, so that the amount ofoxygen vacancies contained in the oxide semiconductor film 208 and theoxide semiconductor film 209 can be reduced. In particular, when thefilm formation temperature of the insulating film 108 or the insulatingfilm 109 which is formed later is set to be high, typically higher than220° C., part of oxygen contained in the oxide semiconductor film 208and the oxide semiconductor film 209 is released and oxygen vacanciesare easily formed.

Further, when the insulating film 109 which is formed later is formedunder the film formation conditions where the amount of defects in thefilm is decreased to increase reliability of the transistor, the amountof oxygen released from the insulating film 109 is easily reduced. Then,it is difficult to fill oxygen vacancies in the oxide semiconductor film208 and the oxide semiconductor film 209 by oxygen supply from theinsulating film 109 in some cases. However, by setting the pressure inthe treatment chamber to be greater than or equal to 100 Pa and lessthan or equal to 250 Pa to reduce damage to the oxide semiconductor film208 and the oxide semiconductor film 209 at the time of forming theinsulating film 108, oxygen vacancies in the oxide semiconductor film208 and the oxide semiconductor film 209 can be reduced even when theamount of oxygen supplied from the insulating film 109 is small.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the insulating film 108 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilm 208 and the oxide semiconductor film 209 can be reduced; thus, thenegative shift in the threshold voltage of the transistor can beinhibited.

In this embodiment, as the insulating film 108, a 50-nm-thick siliconoxynitride film is formed by a plasma CVD method in Which silane anddinitrogen monoxide are used as a source gas. Under the aboveconditions, a silicon oxynitride film which is permeable to oxygen canbe formed.

Next, the insulating film 109 is formed in contact with the insulatingfilm 108. The insulating film 109 is formed using an oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition. Part of oxygen is released by heating fromthe oxide insulating film which contains oxygen at a higher proportionthan the stoichiometric composition. The oxide insulating filmcontaining oxygen at a higher proportion than the stoichiometriccomposition is an oxide insulating film of which the amount of releasedoxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis. Note that the substrate temperature in the TDS analysis ispreferably higher than or equal to 100° C. and lower than or equal to700° C., or higher than or equal to 100° C. and lower than or equal to500° C.

Note that after the insulating film 108 is formed, the insulating film109 is preferably formed in succession without exposure to the air.After the insulating film 108 is formed, the insulating film 109 isformed in succession by adjusting at least one of the flow rate of asource gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 108 and the insulating film 109 can bereduced and oxygen in the insulating film 109 can be moved to the oxidesemiconductor film 208 and the oxide semiconductor film 209;accordingly, the amount of oxygen vacancies in the oxide semiconductorfilm 208 and the oxide semiconductor film 209 can be reduced.

Further, it is preferable that the amount of defects in the insulatingfilm 109 be small, and typically the spin density corresponding to asignal which appears at g=2.001 due to a dangling bond of silicon, belower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 109is provided more apart from the oxide semiconductor film 208 than theinsulating film 108 is; thus, the insulating film 109 may have higherdefect density than the insulating film 108.

A silicon oxide film or a silicon oxynitride film is preferably used asthe insulating film 109. The silicon oxide film or the siliconoxynitride film used as the insulating film 109 can be formed under thefollowing conditions: the substrate placed in a treatment chamber of aplasma CVD apparatus that is vacuum-evacuated is held at a temperaturehigher than or equal to 180° C. and lower than or equal to 280° C.,preferably higher than or equal to 200° C. and lower than or equal to240° C., the pressure in the treatment chamber is greater than or equalto 100 Pa and less than or equal to 250 Pa, preferably greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power higherthan or equal to 0.17 W/cm² and lower than or equal to 0.5 W/cm²,preferably higher than or equal to 0.25 W/cm² and lower than or equal to0.35 W/cm² is supplied to an electrode provided in the treatmentchamber.

The thickness of the insulating film 109 can be greater than or equal to30 nm and less than or equal to 500 nm, preferably greater than or equalto 50 nm and less than or equal to 400 nm.

As the film formation conditions for the insulating film 109, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the insulating film 109 becomes higher than that inthe stoichiometric composition. On the other hand, in the film formed ata substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in the later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Further, the insulating film 108is provided over the oxide semiconductor film 208. Accordingly, in thestep of forming the insulating film 109, the insulating film 108 servesas a protective film for the oxide semiconductor film 208. Consequently,the insulating film 109 can be formed using the high-frequency powerhaving a high power density while damage to the oxide semiconductor film208 is reduced.

Note that in the film formation conditions for the insulating film 109,the flow rate of the deposition gas containing silicon relative to theoxidizing gas can be increased, whereby the amount of defects in theinsulating film 109 can be reduced. Typically, it is possible to form anoxide insulating film in which the amount of defects is small, i.e., thespin density corresponding to a signal which appears at g=2.001 due to adangling bond of silicon is lower than 6×10¹⁷ spins/cm³, preferablylower than or equal to 3×10¹⁷ spins/cm³, more preferably lower than orequal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, thereliability of the transistor can be improved.

In this embodiment, as the insulating film 109, a 400-nm-thick siliconoxynitride film is formed by a plasma CVD method in which silane anddinitrogen monoxide are used as a source gas.

Next, second heat treatment is performed. The heat treatment isperformed typically at a temperature of higher than or equal to 150° C.and lower than or equal to 400° C., preferably higher than or equal to300° C. and lower than or equal to 400° C., more preferably higher thanor equal to 320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for thesecond heat treatment. With the use of an RTA apparatus, the heattreatment can be performed at a temperature of higher than or equal tothe strain point of the substrate if the heating time is short.Therefore, the heat treatment time can be shortened.

The second heat treatment may be performed under an atmosphere ofnitrogen, oxygen, ultra-dry air (air with a water content of 20 ppm orless, preferably 1 ppm or less, more preferably 10 ppb or less), or arare gas (argon, helium, or the like). The atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas preferably does not containhydrogen, water, and the like.

By the second heat treatment, part of oxygen contained in the insulatingfilm 109 can be moved to the oxide semiconductor film 208, so thatoxygen vacancies contained in the oxide semiconductor film 208 can bereduced. Consequently, the amount of oxygen vacancies in the oxidesemiconductor film 208 can be further reduced.

Further, in the case where water, hydrogen, or the like is contained inthe insulating film 108 and the insulating film 109, when the insulatingfilm 110 having a function of blocking water, hydrogen, and the like isformed later and heat treatment is performed, water, hydrogen, or thelike contained in the insulating film 108 and the insulating film 109 ismoved to the oxide semiconductor film 208, so that defects are generatedin the oxide semiconductor film 208. However, by the heating, water,hydrogen, or the like contained in the insulating film 108 and theinsulating film 109 can be released; thus, variation in electricalcharacteristics of the transistor can be reduced, and change inthreshold voltage can be inhibited.

Note that when the insulating film 109 is formed over the insulatingfilm 108 while being heated, oxygen can be moved to the oxidesemiconductor film 208 and oxygen vacancies in the oxide semiconductorfilm 208 can be reduced; thus, the second heat treatment is notnecessarily performed.

Here, heat treatment is performed at 350° C. for one hour in anatmosphere of a mixed gas of nitrogen and oxygen.

Further, when the source electrode 209 a and the drain electrode 209 bare formed, the oxide semiconductor film 208 is damaged by the etchingof the conductive film 220, so that oxygen vacancies are generated onthe back channel side of the oxide semiconductor film 208 (the side ofthe oxide semiconductor film 208 which is opposite the side facing thegate electrode 206). However, with the use of the oxide insulating filmcontaining oxygen at a higher proportion than the stoichiometriccomposition as the insulating film 109, the oxygen vacancies generatedon the back channel side can be repaired by heat treatment. By this,defects contained in the oxide semiconductor film 208 can be reduced,and thus, the reliability of the transistor can be improved.

[Removal of Part of Oxide Insulating Film]

Next, a mask is formed over the insulating film 109 by aphotolithography process, and part of the insulating film 109 and theinsulating film 108 is selectively etched. Thus, an opening 122 isformed over the oxide semiconductor film 209 (see FIG. 5C). At the sametime, another opening which is not illustrated is also formed. Theinsulating film 109 and the insulating film 108 can be etched by a dryetching method, a wet etching method, or both a dry etching method and awet etching method.

After that, second heat treatment may be performed. By the second heattreatment, part of oxygen contained in the insulating film 109 can bemoved to the oxide semiconductor film 208 and the oxide semiconductorfilm 209, so that oxygen vacancies contained in the oxide semiconductorfilm 208 and the oxide semiconductor film 209 can be reduced.

[Formation of Protective Film]

Next, the insulating film 110 is formed (see FIG. 6A). It is possible toprevent outward diffusion of oxygen from the oxide semiconductor film208, the insulating film 108, and the insulating film 109 by using aninsulating film having a blocking effect against oxygen, hydrogen,water, alkali metal, alkaline earth metal, and the like as theinsulating film 110. It is also possible to prevent entry of an impuritysuch as hydrogen or water into the oxide semiconductor film 208 from theoutside. Examples of such an insulating film include nitride insulatingfilms and oxide insulating films such as a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,a gallium oxide film, a gallium oxynitride film, an yttrium oxide film,an yttrium oxynitride film, a hafnium oxide film, and a hafniumoxynitride film.

Note that the structure of the insulating film 110 is not limited to theabove-described structure. A single layer or a stacked layer of aplurality of oxide insulating films and nitride insulating films can beused as appropriate.

In this embodiment, a silicon nitride film containing hydrogen is formedas the insulating film 110.

The oxide semiconductor film 209 is in contact with the insulating film110 in the opening 122. Thus, hydrogen contained in the insulating film110 is diffused into the oxide semiconductor film 209 in the opening 122and bonded to oxygen contained in the oxide semiconductor film 209,whereby electrons serving as carriers are generated. Further, when theinsulating film 110 is formed by a plasma CVD method or a sputteringmethod, the oxide semiconductor film 209 in the opening 122 is exposedto plasma, so that oxygen vacancies are generated in the oxidesemiconductor film 209. When hydrogen contained in the insulating film110 enters the oxygen vacancies, electrons serving as carriers aregenerated. As a result, the conductivity of the oxide semiconductor film209 is increased, so that the oxide semiconductor film 209 becomes aconductive oxide semiconductor film 209. In other words, the conductiveoxide semiconductor film 209 can be referred to as an oxidesemiconductor film with high conductivity or a metal oxide film withhigh conductivity.

Note that when the oxide semiconductor film 209 is exposed to plasmacontaining a rare gas and hydrogen before the insulating film 110 isformed, oxygen vacancies can be formed in the oxide semiconductor film209 and hydrogen can be added to the oxide semiconductor film 209. As aresult, electrons serving as carriers can be further increased in theoxide semiconductor film 209, and the conductivity of the conductiveoxide semiconductor film 209 can be further increased.

The conductive oxide semiconductor film 209 is a film containing a metalelement similar to that of the oxide semiconductor film 208 and containsimpurities. An example of the impurities is hydrogen. As the impurityother than hydrogen, boron, phosphorus, tin, antimony, a rare gaselement, alkali metal, alkaline earth metal, or the like may beincluded.

Both the oxide semiconductor film 208 and the conductive oxidesemiconductor film 209 are formed over the gate insulating film 207 butdiffer in impurity concentration. Specifically, the conductive oxidesemiconductor film 209 has a higher impurity concentration than theoxide semiconductor film 208. For example, the concentration of hydrogencontained in the oxide semiconductor film 208 is lower than 5×10¹⁹atoms/cm³, preferably lower than 5×10¹⁸ atoms cm³, more preferably lowerthan or equal to 1×10¹⁸ atoms/cm³, still more preferably lower than orequal to 5×10¹⁷ atoms/cm³, yet more preferably lower than or equal to1×10¹⁶ atoms/cm³. The concentration of hydrogen contained in theconductive oxide semiconductor film 209 is higher than or equal to8×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰ atoms/cm³,more preferably higher than or equal to 5×10²⁰ atoms/cm³. Theconcentration of hydrogen contained in the conductive oxidesemiconductor film 209 is greater than or equal to 2 times, preferablygreater than or equal to 10 times that in the oxide semiconductor film208.

The conductive oxide semiconductor film 209 has lower resistivity thanthe oxide semiconductor film 208. The resistivity of the oxidesemiconductor film 208 is preferably greater than or equal to 1×10¹times and less than 1×10⁸ times the resistivity of the conductive oxidesemiconductor film 209. The resistivity of the conductive oxidesemiconductor film 209 is typically greater than or equal to 1×10⁻³ Ωcmand less than 1×10⁴ Ωcm, preferably greater than or equal to 1×10⁻³ Ωcm,and less than 1×10⁻¹ Ωcm.

Note that one embodiment of the present invention is not limitedthereto, and it is possible that the conductive oxide semiconductor film209 be not in contact with the insulating film 110 depending oncircumstances.

Further, one embodiment of the present invention is not limited thereto,and the conductive oxide semiconductor film 209 may be formed by adifferent process from that of the oxide semiconductor film 208. In thatcase, the conductive oxide semiconductor film 209 may include adifferent material from that of the oxide semiconductor film 208. Forexample, the conductive oxide semiconductor film 209 may be formed usingindium tin oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide, indiumzinc oxide, indium tin oxide containing silicon oxide, or the like.

In the display device illustrated in this embodiment, one electrode ofthe capacitor is formed at the same time as the semiconductor film ofthe transistor. In addition, a light-transmitting conductive film thatserves as a pixel electrode is used as the other electrode of thecapacitor. Thus, a step of forming another conductive film is not neededto form the capacitor, and the number of steps of manufacturing thesemiconductor device can be reduced. Further, since the pair ofelectrodes of the capacitor has a light-transmitting property, thecapacitor has a light-transmitting property. As a result, the areaoccupied by the capacitor can be increased and the aperture ratio in apixel can be increased.

Next, part of the insulating film 110, the insulating film 109, and theinsulating film 108 is selectively etched. Thus, an opening 142 isformed. At the same time, another opening which is not illustrated isalso formed. The opening 142 and the like can be formed in such a mannerthat a resist mask is formed by a photolithography process and a regionnot covered with the resist mask is etched (see FIG. 6B). At the bottomof the opening 142, a surface of the source electrode 209 a is exposed.

Note that the insulating film 110, the insulating film 109, and theinsulating film 108 can be etched using a dry etching method, forexample. Note that the etching method is not limited thereto, and a wetetching method or a combination of a dry etching method and a wetetching method may be used.

[Formation of Conductive Film]

Next, a conductive film for forming the conductive film 145 is formedover the insulating film 110 so as to cover the opening 122. A resistmask is formed over the conductive film by a photolithography process,and a region not covered with the resist mask is etched to form theconductive film 145 (see FIG. 7A).

At the same time the conductive film 145 is formed, an electrode 226 canbe formed in a region overlapping with the gate electrode 206 and theoxide semiconductor film 208. The conductive film 145 and the electrode226 can be formed using a material and a method similar to those of thegate electrode 206.

When the conductive film 145 is formed using a light-transmittingconductive material, the capacitor 232 can have a light-transmittingproperty. In this embodiment, 100-nm-thick indium tin oxide is used forthe conductive film 145 and the electrode 226 (see FIG. 7A).

The electrode 226 can also serve as a gate electrode. In the case whereone of the gate electrode 206 and the electrode 226 is simply referredto as a “gate electrode”, the other is referred to as a “back gateelectrode” in some cases. One of the gate electrode 206 and theelectrode 226 is referred to as a “first gate electrode”, and the otheris referred to as a “second gate electrode” in some cases.

In general, the back gate electrode is formed using a conductive filmand positioned so that the channel formation region of the semiconductorlayer is positioned between the gate electrode and the back gateelectrode. Thus, the back gate electrode can function in a mannersimilar to that of the gate electrode. The potential of the back gateelectrode may be the same as that of the gate electrode or may be a GNDpotential or a predetermined potential. By changing a potential of theback gate electrode, the threshold voltage of the transistor can bechanged.

Furthermore, the gate electrode and the back gate electrode are formedusing conductive films and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, afunction of blocking static electricity).

In the case where light is incident on the back gate electrode side,when the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

By providing the gate electrode 206 and the electrode 226 so that theoxide semiconductor film 208 is located therebetween, and by setting thepotentials of the gate electrode 206 and the electrode 226 to be thesame, a region of the oxide semiconductor film 208 through whichcarriers flow is enlarged in the film thickness direction; thus, thenumber of transferred carriers is increased. As a result, the on-statecurrent and the field-effect mobility of the transistor are increased.

The gate electrode 206 and the electrode 226 each have a function ofblocking an external electric field; thus, charges in a layer under thegate electrode 206 and in a layer over the electrode 226 do not affectthe oxide semiconductor film 208. Thus, there is little change in thethreshold voltage in a stress test (e.g., a negative gate biastemperature (−GBT) stress test in which a negative voltage is applied toa gate or a +GBT stress test in which a positive voltage is applied to agate). In addition, changes in the rising voltages of on-state currentat different drain voltages can be suppressed. Note that this effect iscaused when the gate electrode 206 and the electrode 226 have the samepotential or different potentials.

The BT stress test is one kind of accelerated test and can evaluate, ina short time, a change in characteristics (i.e., a change over time) oftransistors, which is caused by long-term use. In particular, the amountof change in threshold voltage of the transistor in the BT stress testis an important indicator when examining the reliability of thetransistor. As the amount of change in the threshold voltage in the BTstress test is small, the transistor has higher reliability.

By providing the gate electrode 206 and the electrode 226 and settingthe potentials of the gate electrode 206 and the electrode 226 to be thesame, the amount of change in the threshold voltage is reduced.Accordingly, variation in electrical characteristics among a pluralityof transistors is also reduced.

[Formation of Planarization Film]

Next, the insulating film 211 is formed over the conductive film 145.The insulating film 211 can be formed using a material and a methodsimilar to those of the insulating film 205.

Planarization treatment may be performed on the insulating film 211 toreduce unevenness of a surface on which the light-emitting element 125is formed. The planarization treatment may be, but not particularlylimited to, polishing treatment (e.g., chemical mechanical polishing(CMP)) or dry etching treatment.

Forming the insulating film 211 using an insulating material with aplanarization function can omit polishing treatment. As the insulatingmaterial with a planarization function, for example, an organic materialsuch as a polyimide resin or an acrylic resin can be used. Other thanthe above-described organic materials, it is also possible to use alow-dielectric constant material (low-k material) or the like. Note thatthe insulating film 211 may be formed by stacking a plurality ofinsulating films formed of these materials.

Part of the insulating film 211 in a region overlapping with the opening142 is removed to form an opening 143. At the same time, another openingwhich is not illustrated is also formed. In addition, the insulatingfilm 211 in a region where the external electrode 124 is connected lateris also removed. The opening 143 or the like can be formed in such amanner that a resist mask is formed by a photolithography process overthe insulating film 211 and a region of the insulating film 211 which isnot covered with the resist mask is etched (see FIG. 7B). By formationof the opening 143, the surface of the source electrode 209 a isexposed.

When the insulating film 211 is formed using a photosensitive material,the opening 143 can be formed without the resist mask. In thisembodiment, a photosensitive polyimide resin is used to form theinsulating film 211 and the opening 143.

[Formation of Anode]

Next, the electrode 115 is formed over the insulating film 211 (see FIG.8A). The electrode 115 is preferably formed using a conductive materialthat efficiently reflects light emitted from the EL layer 117 formedlater. Note that the electrode 115 may have a stacked-layer structure ofa plurality of layers without limitation to a single-layer structure.For example, in the case where the electrode 115 is used as an anode, alayer in contact with the EL layer 117 may be a light-transmittinglayer, such as an indium tin oxide layer, having a work function higherthan that of the EL layer 117, and a layer having high reflectance(e.g., aluminum, an alloy containing aluminum, or silver) may beprovided in contact with the layer.

Note that although the display device having a top-emission structure isdescribed as an example in this embodiment, a display device having abottom-emission structure or a dual-emission structure may be used.

In the case of a display device having a bottom-emission structure or adual-emission structure, the electrode 115 may be formed using alight-transmitting conductive material.

The electrode 115 can be formed by forming a conductive film to be theelectrode 115 over the insulating film 211, forming a resist mask by aphotolithography process, and etching a region of the conductive filmwhich is not covered with the resist mask. The conductive film can beetched by a dry etching method, a wet etching method, or both a dryetching method and a wet etching method.

[Formation of Partition]

Next, the partition 114 is formed. The partition 114 is provided toprevent an unintentional electric short-circuit between adjacentlight-emitting elements 125 and unintended light emission therefrom. Inthe case of using a metal mask for formation of the EL layer 117described later, the partition 114 has a function of preventing thecontact of the metal mask with the electrode 115. The partition 114 canbe formed of an organic resin material such as an epoxy resin, anacrylic resin, or an imide resin or an inorganic material such assilicon oxide. The partition 114 is preferably formed so that itssidewall has a tapered shape or a tilted surface with a continuouscurvature. The sidewall of the partition 114 having the above-describedshape enables favorable coverage with the EL layer 117 and the electrode118 formed later.

[Formation of EL Layer]

A structure of the EL layer 117 is described in Embodiment 7.

[Formation of Cathode]

The electrode 118 is used as a cathode in this embodiment, and thus ispreferably formed using a material that has a low work function and caninject electrons into the EL layer 117 described later. As well as asingle-layer of a metal having a low work function, a stack in which ametal material such as aluminum, a conductive oxide material such asindium tin oxide, or a semiconductor material is formed over aseveral-nanometer-thick buffer layer formed of an alkali metal or analkaline earth metal having a low work function may be used as theelectrode 118. As the buffer layer, an oxide of an alkaline earth metal,a halide, a magnesium-silver alloy, or the like can also be used.

In the case where light emitted from the EL layer 117 is extractedthrough the electrode 118, the electrode 118 preferably has a propertyof transmitting visible light. The light-emitting element 125 includesthe electrode 115, the EL layer 117, and the electrode 118.

[Formation of Counter Substrate]

The substrate 121 provided with the light-blocking film 264, thecoloring layer 266, and the overcoat layer 268 (hereinafter simply alsoreferred to as “substrate 121”) is placed over the electrode 118 withthe bonding layer 120 provided therebetween (see FIG. 9). Note that astructure of the substrate 121 is described later.

The bonding layer 120 is in contact with the electrode 118. Thesubstrate 121 is fixed by the bonding layer 120. A light curableadhesive, a reactive curable adhesive, a thermosetting adhesive, or ananaerobic adhesive can be used as the bonding layer 120. For example, anepoxy resin, an acrylic resin, an imide resin, or the like can be used.A drying agent (e.g., zeolite) having a size less than or equal to thewavelength of light or a filler (e.g., titanium oxide or zirconium) witha high refractive index is preferably mixed into the bonding layer 120in the case of a top-emission structure, in which case the efficiency ofextracting light emitted from the EL layer 117 can be improved.

[Separation of Element Formation Substrate]

Next, the element formation substrate 101 attached to the insulatingfilm 205 with the separation layer 113 provided therebetween isseparated from the insulating film 205 (see FIG. 10). As a separationmethod, mechanical force (a separation process with a human hand or agripper, a separation process by rotation of a roller, ultrasonic waves,or the like) may be used. For example, a cut is made in the separationlayer 113 with a sharp edged tool, by laser light irradiation, or thelike and water is injected into the cut. Alternatively, the cut issprayed with a mist of water. A portion between the separation layer 113and the base layer 119 absorbs water through capillarity action, so thatthe element formation substrate 101 can be separated easily.

[Bonding of Substrate]

Next, the substrate 111 is attached to the insulating film 205 with thebonding layer 112 provided therebetween (see FIG. 11). In theabove-described manner, the display device 100 can be manufactured. Thebonding layer 112 can be formed using a material similar to that of thebonding layer 120.

In the above-described manner, the display device 100 can bemanufactured.

In the display device 100 described in this embodiment, part of theinsulating film 108 and the insulating film 109 is removed from a regionnot overlapping with the transistor 431 and the transistor 233. Theremoval of the part of the insulating film 108 and the insulating film109 makes the display device 100 easily bendable. Thus, a highlyflexible display device can be obtained. One embodiment of the presentinvention can provide a highly reliable display device which is noteasily broken even when a bending operation is repeated.

[Structure of Counter Substrate]

Next, a structure that is formed over the substrate 121 provided so asto face the substrate 111 is described below.

First, the substrate 121 is prepared. For the substrate 121, a materialsimilar to that of the substrate 111 can be used. Then, thelight-blocking film 264 is formed over the substrate 121 (see FIG. 12A).After that, the coloring layer 266 is formed (see FIG. 12B).

The light-blocking film 264 and the coloring layer 266 each are formedin a desired position with any of various materials by a printingmethod, an inkjet method, a photolithography method, or the like.

Next, the overcoat layer 268 is formed over the light-blocking film 264and the coloring layer 266 (see FIG. 12C).

For the overcoat layer 268, an organic insulating film of an acrylicresin, an epoxy resin, polyimide, or the like can be used. With theovercoat layer 268, an impurity or the like contained in the coloringlayer 266 can be prevented from diffusing into the light-emittingelement 125 side, for example. Note that the overcoat layer 268 is notnecessarily formed.

Through the above process, the structure formed over the substrate 121can be formed.

[Film Formation Method]

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films which are described in thisembodiment can be formed by a sputtering method or a plasma chemicalvapor deposition (CVD) method, such films may be formed by anothermethod, e.g., a thermal CVD method. A metal organic chemical vapordeposition (MOCVD) method or an atomic layer deposition (ALD) method maybe employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first source gas so thatthe source gases are not mixed, and then a second source gas isintroduced. Note that in the case where the first source gas and theinert gas are introduced at a time, the inert gas serves as a carriergas, and the inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst layer; then the second source gas is introduced to react with thefirst layer; as a result, a second layer is stacked over the firstlayer, so that a thin film is formed. The sequence of the gasintroduction is repeated a plurality of times until a desired thicknessis obtained, whereby a thin film with excellent step coverage can beformed. The thickness of the thin film can be adjusted by the number ofrepetition times of the sequence of the gas introduction; therefore, anALD method makes it possible to accurately adjust a thickness and thusis suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which are described in this embodiment canbe formed by a thermal CVD method such as a MOCVD method or an ALDmethod. For example, in the case where an In—Ga—Zn—O film is formed,trimethylindium, trimethylgallium, and dimethylzinc are used. Note thatthe chemical formula of trimethylindium is In(CH₃)₃. Note that thechemical formula of trimethylgallum is Ga(CH₃)₃. The chemical formula ofdimethylzinc is Zn(CH₃)₂. Without limitation to the above combination,triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead oftrimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can beused instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and a hafnium precursor compound (a hafniumalkoxide solution, typically tetrakis(dimethylamide)hafnium (TDMAH)) areused. Note that the chemical formula of tetrakis(dimethylamide)hafniumis Hf[N(CH₃)₂]₄. Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed using adeposition apparatus employing ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced a plurality of times to form an initial tungstenfilm, and then a WF₆ gas and an H₂ gas are introduced at a time, so thata tungsten film is formed. Note that an SiH₄ gas may be used instead ofa B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced a plurality oftunes to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introducedat a time to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Further, instead of anIn(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gasmay be used.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 2)

A display device 150 having a bottom-emission structure can bemanufactured by modification of the structure of the display device 100having a top-emission structure.

FIG. 13 illustrates a cross-sectional structural example of the displaydevice 150 having a bottom-emission structure. Note that FIG. 13 is across-sectional view of a portion similar to the portion denoted by thedashed-dotted line A1-A2 in FIG. 1A that is a perspective view of thedisplay device 100. The display device 150 having a bottom-emissionstructure differs from the display device 100 in the position where thelight-blocking film 264, the coloring layer 266, and the overcoat layer268 are formed.

In the display device 150, the light-blocking film 264, the coloringlayer 266, and the overcoat layer 268 are formed over the substrate 111.

In the display device 150 having a bottom-emission structure, theelectrode 115 is formed using a light-transmitting conductive material,and the electrode 118 is formed using a conductive material whichefficiently reflects light emitted from the EL layer 117.

In the display device 150, light 235 emitted from the EL layer 117 canbe extracted from the substrate 111 side through the coloring layer 266.

Note that the substrate 111 can be provided with a touch sensor asillustrated in FIG. 23 and as provided in FIG. 21. The substrate 921 canalso be provided with a touch sensor as illustrated in FIG. 24 and asprovided in FIG. 22.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 3)

With a combination of the display device 100 and the display device 150,it is possible to obtain a display device having a dual-emissionstructure in which the light 235 emitted from the EL layer 117 isextracted from both the substrate 111 side and the substrate 121 side.

Note that in the case of the display device having a dual-emissionstructure, both the electrode 115 and the electrode 118 may be formedusing a light-transmitting conductive material.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 4)

In this embodiment, the structure of the transistor 431 described in theabove embodiment is described. FIGS. 14A to 14C illustrate a top viewand cross-sectional views of the transistor 431 as an example of asemiconductor device. The transistor 431 is a channel-etched transistor.Note that the transistor 233 and the transistor 231 can also have astructure similar to that of the transistor 431.

FIG. 14A is a top view of the transistor 431. FIG. 14B is across-sectional view of a portion denoted by a dashed-dotted line X1-X2in FIG. 14A, and FIG. 14C is a cross-sectional view of a portion denotedby a dashed-dotted line Y1-Y2 in FIG. 14A. Note that FIG. 14Billustrates a cross-section in a channel length direction of thetransistor 431, and FIG. 14C illustrates a cross-section in a channelwidth direction of the transistor 431.

The transistor 431 includes the gate electrode 206, the gate insulatingfilm 207, the oxide semiconductor film 208 overlapping with the gateelectrode 206 with the gate insulating film 207 positioned therebetween,and the source electrode 209 a and the drain electrode 209 b in contactwith the oxide semiconductor film 208. A protective film 141 includingthe insulating film 108, the insulating film 109, and the insulatingfilm 110 is formed over the gate insulating film 207, the oxidesemiconductor film 208, and the source electrode 209 a and the drainelectrode 209 b. The electrode 226 overlapping with the oxidesemiconductor film 208 is provided over the insulating film 110.

The transistor 431 has a dual-gate structure in which a plurality ofgate electrodes are included and the oxide semiconductor film 208 isprovided between the plurality of gate electrodes. In the channel widthdirection in FIG. 14C, an end portion of the electrode 226 is positionedon the outer side of the oxide semiconductor film 208. Alternatively, inthe channel width direction, the electrode 226 is provided so as toextend beyond an end portion of the oxide semiconductor film 208 withthe protective film 141 positioned therebetween. Furthermore, in thechannel width direction, the gate electrode 206 faces the electrode 226on the outer side of the oxide semiconductor film 208 with the gateinsulating film 207 and the protective film 141 positioned therebetween.

The positions of end portions of the gate electrode 206, the oxidesemiconductor film 208, and the electrode 226 are described withreference to FIG. 14C.

When the distance between the end portion of the oxide semiconductorfilm 208 and the end portion of the electrode 226 is referred to as adistance d and the thickness of the protective film 141 is referred toas a thickness t, the distance d is preferably smaller than or equal tothe thickness t. The distance d is made smaller than or equal to thethickness t, whereby an electric field of the electrode 226 can exert aninfluence on the end portion of the oxide semiconductor film 208 andthus the whole oxide semiconductor film 208 including its end portioncan serve as a channel.

Defects are formed at the end portion of the oxide semiconductor film208 processed by etching or the like because of damage due to theprocessing, and the end portion of the oxide semiconductor film 208 iscontaminated by attachment of impurities, or the like. Thus, when stresssuch as an electric field is applied, the end portion of the oxidesemiconductor film 208 is easily activated. In other words, the endportion of the oxide semiconductor film processed by etching or the likeeasily becomes n-type (has lower resistance).

When such an unintentional n-type region is in contact with the sourceelectrode 209 a and the drain electrode 209 b, an unintentional current(also referred to as “leakage current”) flows between the sourceelectrode 209 a and the drain electrode 209 b through the region. Inother words, the region serves as a parasitic channel.

However, as illustrated in FIG. 14C, when the end portion of the gateelectrode 206 is positioned on the outer side of the oxide semiconductorfilm 208, generation of a parasitic channel at the side surface of theoxide semiconductor film 208 or in the end portion including the sidesurface and its vicinity is suppressed because of an electric field ofthe gate electrode 206. As a result, the transistor has excellentelectrical characteristics with which drain current is drasticallyincreased when the gate voltage exceeds the threshold voltage.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 5)

In this embodiment, a structure of a transistor 200 which can be used asthe transistor 231, the transistor 233, the transistor 431, and the likeis described with reference to FIGS. 15A to 15C.

FIG. 15A is a top view of the transistor 200. FIG. 15B is across-sectional view of a portion denoted by a dashed-dotted line X3-X4in FIG. 15A, and FIG. 15C is a cross-sectional view of a portion denotedby a dashed-dotted line Y3-Y4 in FIG. 15A. Note that FIG. 15Billustrates a cross-section in a channel length direction of thetransistor 200, and FIG. 15C illustrates a cross-section in a channelwidth direction of the transistor 200.

The transistor 200 can be manufactured in a manner similar to that ofthe transistor 431 described in the above embodiment. Note that thetransistor 200 differs from the transistor 431 in the shapes of theoxide semiconductor film 208 and the gate electrode 206.

In the transistor 200, the end portion of the gate electrode 206 islocated on the outer side of the end portion of the oxide semiconductorfilm 208 not only in the channel width direction but also in the channellength direction (see FIG. 15B). When the gate electrode 206 is providedsuch that the end portion of the gate electrode 206 is located on theouter side of the end portion of the oxide semiconductor film 208, avariation in electrical characteristics of the transistor due to lightirradiation can be further suppressed.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 6)

In this embodiment, a structure of a transistor 250 which can be used asthe transistor 231, the transistor 233, the transistor 431, and the likeis described with reference to FIGS. 16A to 16D.

FIG. 16A is a top view of the transistor 250. FIG. 16B is across-sectional view of a portion denoted by a dashed-dotted line X5-X6in FIG. 16A, and FIG. 16C is a cross-sectional view of a portion denotedby a dashed-dotted line Y5-Y6 in FIG. 16A. FIG. 16D is an enlarged viewof a portion 290 in FIG. 16B. Note that FIG. 16B illustrates across-section in a channel length direction of the transistor 250, andFIG. 16C illustrates a cross-section in a channel width direction of thetransistor 250.

The transistor 250 can be manufactured in a manner similar to that ofthe transistor 431 described in the above embodiment. Note that in thetransistor 250, an oxide semiconductor film 218 is formed in contactwith the oxide semiconductor film 208. Although the electrode 226serving as a back gate electrode is not provided in the transistor 250,it is needless to say that the electrode 226 may be provided asnecessary.

The oxide semiconductor film 218 is an oxide film containing one or moreelements which form the oxide semiconductor film 208. Thus, interfacescattering is unlikely to occur at the interface between the oxidesemiconductor film 208 and the oxide semiconductor film 218. Thus, thetransistor can have a high field-effect mobility because the movement ofcarriers is not hindered at the interface.

The oxide semiconductor film 218 typically contains In—Ga oxide, In—Znoxide, or In—M—Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd). Theenergy at the conduction band bottom of the oxide semiconductor film 218is closer to a vacuum level than that of the oxide semiconductor film208 is, and typically, the difference between the enemy at theconduction band bottom of the oxide semiconductor film 218 and theenergy at the conduction band bottom of the oxide semiconductor film 208is any one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, and 0.15eV or more and any one of 2 eV or less, 1 eV or less, 0.5 eV or less,and 0.4 eV or less. That is, the difference between the electronaffinity of the oxide semiconductor film 218 and the electron affinityof the oxide semiconductor film 208 is any one of 0.05 eV or more, 0.07eV or more, 0.1 eV or more, and 0.15 eV or more and any one of 2 eV orless, 1 eV or less, 0.5 eV or less, and 0.4 eV or less.

The oxide semiconductor film 218 preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 218 contains a larger amount of Al,Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in anatomic ratio, any of the following effects may be obtained:

(1) the energy gap of the oxide semiconductor film 218 is widened;

(2) the electron affinity of the oxide semiconductor film 218 decreases;

(3) an impurity from the outside is blocked;

(4) an insulating property increases as compared to the oxidesemiconductor film 208; and

(5) oxygen vacancies are less likely to be generated in the oxidesemiconductor film 218 containing a larger amount of Ga, Y, Zr, La, Ce,or Nd in an atomic ratio than the amount of In in an atomic ratiobecause Ga, Y, Zr, La, Ce, or Nd is a metal element which is stronglybonded to oxygen.

In the case where the oxide semiconductor film 218 contains an In—M—Znoxide, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is less than 50 atomic % and the atomic percentage of M is greaterthan or equal to 50 atomic %, or more preferably, the atomic percentageof In is less than 25 atomic % and the atomic percentage of M is greaterthan or equal to 75 atomic %.

Further, in the case where each of the oxide semiconductor film 208 andthe oxide semiconductor film 218 contains an In—M—Zn oxide (M representsGa, Y, Zr, La, Ce, or Nd), the proportion of M (M represents Ga, Y, Zr,La, Ce, or Nd) in the oxide semiconductor film 218 is higher than thatin the oxide semiconductor film 208. Typically, the proportion of M inthe oxide semiconductor film 218 is 1.5 or more times, twice or more, orthree or more times as high as that in the oxide semiconductor film 208.

Furthermore, in the case where each of the oxide semiconductor film 208and the oxide semiconductor film 218 contains an In—A—Zn oxide (Mrepresents Al, Ga, Y, Zr, La, Ce, or Nd), when In:M:Zn=x₁:y₁:z₁ [atomicratio] is satisfied in the oxide semiconductor film 218 andIn:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfied in the oxide semiconductorfilm 208, y₁/x₁ is higher than y₂/x₂, or preferably y₁/x₁ is 1.5 or moretimes as high as y₂/x₂. More preferably, y₁/x₁ is twice or more as highas y₂/x₂, or still more preferably y₁/x₁ is three or more times as highas y₂/x₂. In this case, it is preferable that in an oxide semiconductorfilm, y₂ be higher than or equal to x₂ because a transistor includingthe oxide semiconductor film can have stable electrical characteristics.However, when y₂ is larger than or equal to three or more times x₂, thefield-effect mobility of the transistor including the oxidesemiconductor film is reduced. Accordingly, y₂ is preferably smallerthan three times x₂.

In the case where the oxide semiconductor film 208 contains an In—M—Znoxide (M represents Ga, Y, Zr, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for formingthe oxide semiconductor film 208, x₁/y₁ is preferably greater than orequal to ⅓ and less than or equal to 6, further preferably greater thanor equal to 1 and less than or equal to 6, and z₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film is easily formed as the oxide semiconductor film 208.Typical examples of the atomic ratio of the metal elements of the targetare In:M:Zn=1:1:1, In:M:Zn=5:5:6, In:M:Zn=2:1:2, In:M:Zn=3:1:2, and thelike.

In the case where the oxide semiconductor film 218 contains an In—M—Znoxide (M represents Ga, Y Zr, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for formingthe oxide semiconductor film 218, x₂/y₂ is preferably less than x₁/y₁,and z₂/y₂ is preferably greater than or equal to ⅓ and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₂/y² is greater than or equal to 1 andless than or equal to 6, a CAAC-OS film is easily formed as the oxidesemiconductor film 218. Typical examples of the atomic ratio of themetal elements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4,In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor film 208 and the oxide semiconductorfilm 218 varies within a range of ±40% of that in the above atomic ratioas an error.

The thickness of the oxide semiconductor film 218 is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 218 may each have a non-single-crystalstructure, for example, like the oxide semiconductor film 208. Note thatthe non-single-crystal structure includes a CAAC structure, apolycrystalline structure, a microcrystalline structure, or an amorphousstructure.

The oxide semiconductor film 218 may have an amorphous structure, forexample. An amorphous oxide semiconductor film, for example, hasdisordered atomic arrangement and no crystalline component.Alternatively, the oxide film having an amorphous structure have, forexample, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor films 208 and 218 may each be a mixedfilm including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a region having a CAACstructure, and a region having a single crystal structure. The mixedfilm includes, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a region having a CAAC structure, and aregion having a single crystal structure in some cases. Further, themixed film has a layered structure of two or more of a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a region having a CAACstructure, and a region having a single crystal structure in some cases.

Here, the oxide semiconductor film 218 is provided between the oxidesemiconductor film 208 and the insulating film 108. Hence, if trapstates are formed between the oxide semiconductor film 218 and theinsulating film 108 owing to impurities and defects, electrons flowingin the oxide semiconductor film 208 are less likely to be captured bythe trap states because there is a distance between the trap states andthe oxide semiconductor film 208. Accordingly, the amount of on-statecurrent of the transistor can be increased, and the field-effectmobility can be increased. When electrons are captured by the trapstates, the electrons become negative fixed charges. As a result, athreshold voltage of the transistor changes. However, by the distancebetween the oxide semiconductor film 208 and the trap states, capture ofthe electrons by the trap states can be reduced, and accordingly achange of the threshold voltage can be reduced.

Further, impurities from the outside can be blocked by the oxidesemiconductor film 218, and accordingly, the amount of impurities whichmove from the outside to the oxide semiconductor film 208 can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 218. Consequently, the impurity concentrationand the amount of oxygen vacancies in the oxide semiconductor film 208can be reduced.

Note that the oxide semiconductor film 208 and the oxide semiconductorfilm 218 are not formed by simply stacking each film, but are formed toform a continuous junction (here, in particular, a structure in whichthe energy of the bottom of the conduction band is changed continuouslybetween the films). In other words, a stacked-layer structure in whichthere exists no impurity which forms a defect level such as a trapcenter or a recombination center at each interface is provided. If animpurity exists between the oxide semiconductor film 208 and the oxidesemiconductor film 218 which are stacked, a continuity of the energyband is damaged, and the carrier is captured or recombined at theinterface and then disappears.

In order to form such a continuous junction, it is necessary to formfilms continuously without being exposed to air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber of the sputtering apparatus ispreferably evacuated to a high vacuum (to the degree of about 5×10⁻⁷ Pato 1×10⁻⁴ Pa) by an adsorption vacuum pump such as a cryopump so thatwater and the like acting as impurities for the oxide semiconductor filmare removed as much as possible. Alternatively, a combination of a turbomolecular pump and a cold trap is preferably used to prevent back-flowof a gas, especially a gas containing carbon or hydrogen, from anexhaust system into a chamber.

In the step of forming the source electrode 209 a and the drainelectrode 209 b, a surface of the oxide semiconductor film 218 may beetched.

[Band Structure Diagram]

FIG. 17 is a schematic diagram illustrating a band structure of aportion denoted by a dashed-dotted line Z1-Z2 in FIG. 16D. In FIG. 17,EcI1 denotes the energy of the bottom of the conduction band in the gateinsulating film 207; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 208; EcS2 denotes theenergy of the bottom of the conduction band in the oxide semiconductorfilm 218; and EcI2 denotes the energy of the bottom of the conductionband in the insulating film 108.

As illustrated in FIG. 17, in the junction portion between the oxidesemiconductor films 208 and 218, the conduction band minimums thereofsmoothly vary. In other words, the conduction band minimums arecontinuous. This is because the oxide semiconductor films 208 and 218contain a common metal element and oxygen is transferred between theoxide semiconductor films 208 and 218, so that a mixed layer is formed.

As shown in FIG. 17, the oxide semiconductor film 208 serves as a welland a channel region is formed in the oxide semiconductor film 208. Notethat since the energy of the bottom of the conduction band of the oxidesemiconductor films 208 and 218 is continuously changed, it can be saidthat the oxide semiconductor films 208 and 218 are continuous.

In the step of forming the source electrode 209 a and the drainelectrode 209 b, a surface of the oxide semiconductor film 218 may beetched. Thus, trap states resulting from impurities or defects can beformed in the vicinity of the interface between the oxide semiconductorfilm 218 and the insulating film 108. However, since the oxidesemiconductor film 218 is provided, the oxide semiconductor film 208 canbe distanced from the trap states. However, in the case where the energydifference (dEcS) between EcS1 and EcS2 is small, electrons in the oxidesemiconductor film 208 may reach the trap level by passing over theenergy gap. Since the electron is trapped at the trap level, a negativefixed charge is generated at the interface with the insulating film,causing the threshold voltage of the transistor to be shifted in thepositive direction. Therefore, the energy difference (dEcS) between EcS1and EcS2 is preferably set to be larger than or equal to 0.1 eV, morepreferably larger than or equal to 0.15 eV, in which case a change inthe threshold voltage of the transistor can be reduced and stableelectrical characteristics can be obtained.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 7)

In this embodiment, structure examples of a light-emitting element thatcan be applied to the light-emitting element 125 are described. Notethat an EL layer 320 described in this embodiment corresponds to the ELlayer 117 described in the above embodiment.

<Structure of Light-Emitting Element>

In a light-emitting element 330 illustrated in FIG. 18A, the EL layer320 is interposed between a pair of electrodes (an electrode 318 and anelectrode 322). Note that the electrode 318 is used as an anode and theelectrode 322 is used as a cathode as an example in the followingdescription of this embodiment.

The EL layer 320 includes at least a light-emitting layer and may have astacked-layer structure including a functional layer other than thelight-emitting layer. As the functional layer other than thelight-emitting layer, a layer containing a substance having a highhole-injection property, a substance having a high hole-transportproperty, a substance having a high electron-transport property, asubstance having a high electron-injection property, a bipolar substance(a substance having high electron- and hole-transport properties), orthe like can be used. Specifically, functional layers such as ahole-injection layer, a hole-transport layer, an electron-transportlayer, and an electron-injection layer can be used in combination asappropriate.

The light-emitting element 330 illustrated in FIG. 18A emits light whencurrent flows because of a potential difference generated between theelectrode 318 and the electrode 322 and holes and electrons arerecombined in the EL layer 320. That is, the light-emitting region isformed in the EL layer 320.

In the present invention, light emitted from the light-emitting element330 is extracted to the outside from the electrode 318 side or theelectrode 322 side. Therefore, one of the electrode 318 and theelectrode 322 is formed of a light-transmitting substance.

Note that a plurality of EL layers 320 may be stacked between theelectrode 318 and the electrode 322 as in a light-emitting element 331illustrated in FIG. 18B. In the case where n (n is a natural number of 2or more) layers are stacked, a charge generation layer 320 a ispreferably provided between an m-th EL layer 320 and an (m+1)-th ELlayer 320. Note that in is a natural number greater than or equal to 1and less than n.

The charge generation layer 320 a can be formed using a compositematerial of an organic compound and a metal oxide, a metal oxide, acomposite material of an organic compound and an alkali metal, analkaline earth metal, or a compound thereof; alternatively, thesematerials can be combined as appropriate. Examples of the compositematerial of an organic compound and a metal oxide include compositematerials of an organic compound and a metal oxide such as vanadiumoxide, molybdenum oxide, and tungsten oxide. As the organic compound, avariety of compounds can be used; for example, low molecular compoundssuch as an aromatic amine compound, a carbazole derivative, and aromatichydrocarbon and oligomers, dendrimers, and polymers of these lowmolecular compounds. As the organic compound, it is preferable to usethe organic compound which has a hole-transport property and has a holemobility of 10⁻⁶ cm²/Vs or higher. However, substances other than thesubstances given above may also be used as long as the substances havehigher hole-transport properties than electron-transport properties.These materials used for the charge generation layer 320 a haveexcellent carrier-injection properties and carrier-transport properties;thus, the light-emitting element 330 can be driven with low current andwith low voltage.

Note that the charge generation layer 320 a may be formed with acombination of a composite material of an organic compound and a metaloxide with another material. For example, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a layer containing a compound of a substance selected fromsubstances with an electron-donating property and a compound with a highelectron-transport property. Moreover, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a transparent conductive film.

The light-emitting element 331 having such a structure is unlikely tohave problems such as energy transfer and quenching and has an expandedchoice of materials, and thus can easily have both high emissionefficiency and a long lifetime. Moreover, it is easy to obtainphosphorescence from one light-emitting layer and fluorescence from theother light-emitting layer.

The charge generation layer 320 a has a function of injecting holes toone of the EL layers 320 that is in contact with the charge generationlayer 320 a and a function of injecting electrons to the other EL layer320 that is in contact with the charge generation layer 320 a, whenvoltage is applied between the electrode 318 and the electrode 322.

The light-emitting element 331 illustrated in FIG. 18B can provide avariety of emission colors by changing the type of the light-emittingsubstance used for the EL layer 320. In addition, a plurality oflight-emitting substances emitting light of different colors may be usedas the light-emitting substances, whereby light emission having a broadspectrum or white light emission can be obtained.

In the case of obtaining white light emission using the light-emittingelement 331 illustrated in FIG. 18B, as for the combination of aplurality of EL layers, a structure for emitting white light includingred light, green light, and blue light may be used; for example, thestructure may include a light-emitting layer containing a bluefluorescent substance as a light-emitting substance and a light-emittinglayer containing red and green phosphorescent substances aslight-emitting substances. Alternatively, a structure including alight-emitting layer emitting red light, a light-emitting layer emittinggreen light, and a light-emitting layer emitting blue light may beemployed. Further alternatively, with a structure includinglight-emitting layers emitting light of complementary colors, whitelight emission can be obtained. In a stacked-layer element including twolight-emitting layers in which light emitted from one of thelight-emitting layers and light emitted from the other light-emittinglayer have complementary colors to each other, the combinations ofcolors are as follows: blue and yellow, blue-green and red, and thelike.

Note that in the structure of the above-described stacked-layer element,by providing the charge generation layer between the stackedlight-emitting layers, the element can have a long lifetime in ahigh-luminance region while keeping the current density low. Inaddition, the voltage drop due to the resistance of the electrodematerial can be reduced, whereby uniform light emission in a large areais possible.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

(Embodiment 8)

In this embodiment, examples of an electronic device and a lightingdevice including the display device of one embodiment of the presentinvention are described with reference to drawings.

As examples of electronic devices with flexibility, the following can begiven: television devices (also called televisions or televisionreceivers), monitors of computers or the like, digital cameras, digitalvideo cameras, digital photo frames, mobile phones (also called cellularphones or mobile phone devices), portable game machines, portableinformation terminals, audio reproducing devices, large game machinessuch as pachinko machines, and the like.

In addition, a lighting device or a display device can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

FIG. 19A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using the display device in the display portion 7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 19A is touched with a finger or the like, data can be input to themobile phone 7400. Further, operations such as making a call andinputting a character can be performed by touch on the display portion7402 with a finger or the like.

The power can be turned on or off with the operation button 7403. Inaddition, types of images displayed on the display portion 7402 can beswitched; for example, switching images from a mail creation screen to amain menu screen is performed with the operation button 7403.

Here, the display portion 7402 includes the display device of oneembodiment of the present invention. Thus, the mobile phone can have acurved display portion and high reliability.

FIG. 19B is an example of a wristband-type display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a sending and receiving device 7104.

The portable display device 7100 can receive a video signal with thesending and receiving device 7104 and can display the received video onthe display portion 7102. In addition, with the sending and receivingdevice 7104, the portable display device 7100 can send an audio signalto another receiving device.

With the operation button 7103, power ON/OFF, switching displayedvideos, adjusting volume, and the like can be performed.

Here, the display portion 7102 includes the display device of oneembodiment of the present invention. Thus, the portable display devicecan have a curved display portion and high reliability.

FIGS. 19C to 19E illustrate examples of lighting devices. Lightingdevices 7200, 7210, and 7220 each include a stage 7201 provided with anoperation switch 7203 and a light-emitting portion supported by thestage 7201.

The lighting device 7200 illustrated in FIG. 19C includes alight-emitting portion 7202 with a wave-shaped light-emitting surfaceand thus is a good-design lighting device.

A light-emitting portion 7212 included in the lighting device 7210illustrated in FIG. 19D has two convex-curved light-emitting portionssymmetrically placed. Thus, light radiates from the lighting device7210.

The lighting device 7220 illustrated in FIG. 19E includes aconcave-curved light-emitting portion 7222. This is suitable forilluminating a specific range because light emitted from thelight-emitting portion 7222 is collected to the front of the lightingdevice 7220.

The light-emitting portion included in each of the lighting devices7200, 7210, and 7220 is flexible; thus, the light-emitting portion maybe fixed on a plastic member, a movable frame, or the like so that anemission surface of the light-emitting portion can be bent freelydepending on the intended use.

The light-emitting portions included in the lighting devices 7200, 7210,and 7220 each include the display device of one embodiment of thepresent invention. Thus, the lighting devices can have curved displayportions and high reliability.

FIG. 20A illustrates an example of a portable display device. A displaydevice 7300 includes a housing 7301, a display portion 7302, operationbuttons 7303, a display portion pull 7304, and a control portion 7305.

The display device 7300 includes a rolled flexible display portion 7302in the cylindrical housing 7301.

The display device 7300 can receive a video signal with the controlportion 7305 and can display the received video on the display portion7302. In addition, a battery is included in the control portion 7305.Moreover, a connector may be included in the control portion 7305 sothat a video signal or power can be supplied directly.

With the operation buttons 7303, power ON/OFF, switching of displayedvideos, and the like can be performed.

FIG. 20B illustrates a state in which the display portion 7302 is pulledout with the display portion pull 7304. Videos can be displayed on thedisplay portion 7302 in this state. Further, the operation buttons 7303on the surface of the housing 7301 allow one-handed operation.

Note that a reinforcement frame may be provided for an edge portion ofthe display portion 7302 in order to prevent the display portion 7302from being curved when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

The display portion 7302 includes the display device of one embodimentof the present invention. Thus, the display portion 7302 is a flexible,highly reliable display device, which makes the display device 7300lightweight and highly reliable.

It is needless to say that one embodiment of the present invention isnot limited to the above-described electronic devices and lightingdevices as long as the display device of one embodiment of the presentinvention is included.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in other embodiments.

This application is based on Japanese Patent Application serial no.2013-146046 filed with Japan Patent Office on Jul. 12, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a flexiblesubstrate; a first gate electrode over the flexible substrate; a firstinsulating film over the first gate electrode; an oxide semiconductorfilm over the first insulating film, the oxide semiconductor filmoverlapping with the first gate electrode; a source electrode or a drainelectrode in contact with a top surface of the oxide semiconductor film;a second insulating film and a third insulating film over the oxidesemiconductor film; a second gate electrode over the third insulatingfilm, the second gate electrode overlapping with the oxide semiconductorfilm; and a capacitor, wherein the capacitor comprises a firstelectrode, a second electrode over the first electrode, and a fourthinsulating film between the first electrode and the second electrode,wherein the first electrode comprises a first region in contact with thefourth insulating film, wherein the first electrode is provided on asame surface as the oxide semiconductor film, wherein the firstelectrode comprises a same material as the oxide semiconductor film,wherein the fourth insulating film comprises a same material as thethird insulating film, and wherein the second electrode comprises a samematerial as the second gate electrode.
 2. The display device accordingto claim 1, wherein an opening portion of the second insulating filmcomprises a second region overlapping with the first region.
 3. Thedisplay device according to claim 1, further comprising an organic filmover the second gate electrode and the first region.
 4. The displaydevice according to claim 1, wherein the oxide semiconductor filmcomprises indium, gallium, and zinc.
 5. The display device according toclaim 1, wherein the first electrode comprises indium, gallium, andzinc.
 6. The display device according to claim 1, wherein the firstelectrode comprises hydrogen.
 7. The display device according to claim1, wherein the third insulating film is in contact with a side surfaceof the second insulating film.
 8. The display device according to claim1, wherein the second gate electrode comprises a same material as thesecond electrode.
 9. A display device comprising: a flexible substrate;a first gate electrode over the flexible substrate; a first insulatingfilm over the first gate electrode; an oxide semiconductor film over thefirst insulating film, the oxide semiconductor film overlapping with thefirst gate electrode; a source electrode or a drain electrode in contactwith a top surface of the oxide semiconductor film; a second insulatingfilm and a third insulating film over the oxide semiconductor film; asecond gate electrode over the third insulating film, the second gateelectrode overlapping with the oxide semiconductor film; alight-emitting element electrically connected with the source electrodeor the drain electrode; and a capacitor, wherein the capacitor comprisesa first electrode, a second electrode over the first electrode, and afourth insulating film between the first electrode and the secondelectrode, wherein the first electrode comprises a first region incontact with the fourth insulating film, wherein the first electrode isprovided on a same surface as the oxide semiconductor film, wherein thefirst electrode comprises a same material as the oxide semiconductorfilm, wherein the fourth insulating film comprises a same material asthe third insulating film, wherein the second electrode comprises a samematerial as the second gate electrode, and wherein the light-emittingelement comprises a region overlapping with the capacitor.
 10. Thedisplay device according to claim 9, wherein an opening portion of thesecond insulating film comprises a second region overlapping with thefirst region.
 11. The display device according to claim 9, furthercomprising an organic film over the second gate electrode and the firstregion.
 12. The display device according to claim 9, wherein the oxidesemiconductor film comprises indium, gallium, and zinc.
 13. The displaydevice according to claim 9, wherein the first electrode comprisesindium, gallium, and zinc.
 14. The display device according to claim 9,wherein the first electrode comprises hydrogen.
 15. The display deviceaccording to claim 9, wherein the third insulating film is in contactwith a side surface of the second insulating film.
 16. The displaydevice according to claim 9, wherein the second gate electrode comprisesa same material as the second electrode.
 17. A display devicecomprising: a transistor and a capacitor, wherein the transistorcomprises: a first gate electrode; a first insulating film over thefirst gate electrode; an oxide semiconductor film over the firstinsulating film, the oxide semiconductor film overlapping with the firstgate electrode; a source electrode or a drain electrode in contact witha top surface of the oxide semiconductor film; a second insulating filmand a third insulating film over the oxide semiconductor film; and asecond gate electrode over the third insulating film, the second gateelectrode overlapping with the oxide semiconductor film, wherein thecapacitor comprises: a first electrode over the first insulating film; asecond electrode over the first electrode; and a fourth insulating filmbetween the first electrode and the second electrode, wherein the firstelectrode comprises a first region in contact with the fourth insulatingfilm, wherein the first electrode is provided on a same surface as theoxide semiconductor film, wherein the first electrode comprises a samematerial as the oxide semiconductor film, wherein the fourth insulatingfilm comprises a same material as the third insulating film, wherein thesecond electrode comprises a same material as the second gate electrode,and wherein an end portion of the second gate electrode is located on anouter side of an end portion of the oxide semiconductor film in achannel width direction.
 18. The display device according to claim 17,wherein an opening portion of the second insulating film comprises asecond region overlapping with the first region.
 19. The display deviceaccording to claim 17, further comprising an organic film over thesecond gate electrode and the first region.
 20. The display deviceaccording to claim 17, further comprising a light-emitting elementelectrically connected with the source electrode or the drain electrode,wherein the light-emitting element comprises a region overlapping withthe capacitor.